Also been looking forward to this one, since I tend to prefer fantasy books that have adult protagonists nowadays.

1) Don’t automatically turn adults into teenagers.

I started reading a story recently which started with a monologue (yes, I know, first warning sign) in which the character complained about not having training, not having nice clothes, being worked like a servant by her older sister, scratching her hands on the berry bushes, and not being able to watch a parade pass. From all of this, I assumed the character was about 13.
Then I got to the part where the character revealed, via more monologue, that she was 31.
My gods.
Yeah, I know that many people succumb to the temptation to write fantasy stories about someone stuck in a place she can’t wait to get away from, and then have that character snatched up by passing adventure. That’s one thing with teenagers, since most fantasy societies are set up to give little power to younger people, and they mostly don’t have the experience or the training to help themselves out of a sticky situation.
I expect much, much more of an adult. An adult character, even one trapped in a situation she finds it hard to leave, should have much deeper reasons for staying there than the teenager. For one thing, why doesn’t she have the power to move out or move on? What circumstances conspire to force her to stay there? There might be an explanation, but you can’t rely on the automatic, unspoken teenage reasons. (No, you can’t. I don’t care what good reasons are in your head. If they’re not in the story, I am going to laugh at an adult with nothing holding her back but her own wimpiness and put the book down).
For a second thing, wouldn’t her society look a little askance at an adult still staying with her parents for no real reason? It’s one thing if she takes care of them, one thing if she’s considered “unmarriageable,” or one thing if some physical disability prevents her from getting out of there. But most fantasy societies aren’t set up so that a thirty-one-year old woman on the road would immediately get sent back to Mommy and Daddy.
You will have to give adults reasons for staying home that teenagers don’t have, unless you have damn good reasons. And if you have damn good reasons, put them in the story, so that it becomes a damn good story instead of a damn pathetic one.

2) Adults can have multiple Incidents in their lives.

Teenagers might have had time for only one defining Incident, like the time their brother died of fever, or their Incident might be what starts the story off, like the discovery of magic. But I am so fucking sick of adult characters who had one Incident in their entire lives, such as the death of a beloved parent, that froze their psychology and turned them into walking automatons focused on the Incident. Any book that involves that now will get sent to the used book store.
I know that a lot of people think characters brooding on their single trauma twenty years later is romantic—though I can’t figure out why, and believe me, I’ve tried—but it just isn’t normal. If your character is still having nightmares about one single Incident twenty years later, he has deep psychological problems, okay? Do not try and tell me that this person is one who just needs the right quest or true love or magic power to cheer him up. If he’s that fixated, it’ll take a lot more healing.
It’s simply obnoxious to write an adult character like this. So just one important thing happened to them in twenty, twenty-five, thirty years of life? And they fixated on it and didn’t move past it? And the author wants me to believe that this person is a tortured soul, but also a brilliant, empathic, perceptive, fascinating, wonderful character? Uh-huh. I would think it far more likely that this adult would be a tortured soul who couldn’t see past his own torture to look at other people’s problems. He would need an awful lot of help before he could help anyone else.
Hey, if you do want to write a fantasy story of psychological healing, that’s fine. That’s great, in fact, since so many of them ignore the healing process altogether. But do not dump this load of horseshit on my plate and expect me to eat it.

3) Adults have a reason to possess more skills than the average teenager.

I really like reading fantasy novels that open with a depiction of a character doing something competent, as well as clever/strong/brave/daring/really cool. (All of those at once is even better). And if the character’s an adult, there’s no warning bell of the Genius Teenager in my mind. I am perfectly willing to believe that someone who’s managed to survive in a violent fantasy world for thirty years got himself sword-training at some point. And he can protect other people, and duel other experienced swordsmen, and remember his training with a wince and a smile, and I won’t have a qualm. At least there was training there.
This is probably the area where adult protagonists are of the greatest value to the average fantasy. Most average fantasy authors want to write a) lots of exciting action scenes in which b) their hero is the one performing the action. However, they also want to write c) a teenager who d) grows in skill along the fantasy journey.
These plotlines are not that compatible. Try to combine them in the same story without explanation, and they’ll squabble like two cats in Schrödinger’s box. You will have no need to open the box, because it won’t matter which plotline survives; the story will be dead on arrival, I assure you.
An adult protagonist privileges the first plotline at the expense of the second, that’s true. Good. I am much more interested in reading about people who have reason to be out there on the road trying to save the world than I am in reading yet another tale of a teenager who gets pulled along for something he or she didn’t earn (inborn magical powers, bloodline, random prophecy-declared Destiny) and will grow into without fuss. I want fuss. I like fuss. And not the fuss of fighting with parents and figuring out character romances, either, which tend to be two big teenage sources of it. Adult characters can do lots of really cool things, and even get better as the story goes along, and the only thing that you need to do is ignore the temptation to write yet another very average bildungsroman (the fancy term for a coming-of-age story).

4) Adults are often more responsible than teenagers.

No, not all the time. That’s why the “often” is there.
But I feel like I can trust an adult character, if he needs to run out the door in pursuit of something or other, to bring along some food, an extra change of clothes, a bedroll, and other practical things, whereas the teenager is much more likely to bring along her stuffed bear and the mystical amulet but forget the food. If an adult character is trapped in a hostile town, he may have the sense to hold his tongue, while the teenager will blurt out some “witty” nonsense, under the impression that these people are exactly like the bullies he had experience with in his hometown. An adult character who’s a warrior will have killed before and will do it again, though probably with a twinge of regret. This spares me from having to read about yet another scene where a teenager kills for the first time, throws up, and is reassured by his mentor that it was self-defense, or destiny, or what-the-fuck-ever.
These minor crises are the bread-and-butter of typical adolescent fantasy. They’re part of the growing-up story. But I would like to read about characters who have already encountered them and grown past them, please. That way, they can get on to the major crises. (The rant on “putting your characters through absolute fucking hell” will be so much fun to write).

5) The audience will expect that adults make better philosophers than teenagers.

Fantasy teenagers are a mercurial breed. Does the author need them to believe in the first part of the novel that elves are evil, and change their minds in the second half, when they meet one good elf? Not a problem! Does the author need them to swallow hook, line, and sinker the ridiculous half-explanation that many a Wise Old Mentor winks and smiles about? Done and said! Does the author need them to switch their emotions depending on the plot? The author’s already there! After all, everyone knows that teenagers always sacrifice their most deeply-held beliefs at the drop of a hat, and if one of your readers doubts you, then you can always blame it on hormones.
You cannot pull this shit nearly as well with adult characters, mostly because readers will stand up and snarl at you. People notice, yes indeed they do, when the woman who’s believed that elves are evil for forty years just drops her hostility and welcomes them into her house because an elf smiles at her. They notice when adult characters switch their emotions on and off as the plot requires. They notice when the old man who’s been crusty for 300 pages just breaks down and cries with no explanation.
Working with stereotypes? Sure, it is. But if fantasy authors can get away with claiming that every teenager is the rebellious sulky changeable type (I wasn’t) and have most readers accept it because these are growing-up stories and the characters can be wise and settled later, then I think most readers would be willing to accept that adults have more experience to draw on, and therefore are likely to come to psychological epiphanies in different ways.
In some cases, that’ll be sheer life experience. The forty-year-old woman has believed for forty years that elves are evil, more than twice as long as the typical fantasy teenager has lived. So now these elves are pounding at her door? How does she know that it’s not an evil elven plot? She could weigh an experience of sweet words and a smile against having watched five friends die at the end of elven arrows, but it would still be a weighing of experience, not just a simpering reaction of, “Oh, well, if you say you’re not evil, it must be true!” that adults often inflict on fantasy teenagers.
In other cases, adult characters will have more education than teenagers could ever hope to have, more insight into the human heart, more political experience. (Authors often have a pathological aversion to letting teenagers be experienced in court politics—even if the teenager’s grown up in the court, she’ll still be perky and innocent—but adults are fine). They’ll also have had a chance, often, to move out from under their parents’ shadows. Best of all, most authors don’t even think to use that stupid-ass excuse that everything wrong with their behavior is due to their hormones.
Try philosophy and stories of transformation with adults. Overturning someone’s whole belief system when it’s been established for twenty years makes for a more moving story, I think, than altering the beliefs of someone who changes his mind at the author’s whim.
(I think I will do a rant on transformative fantasy, as well).

6) Adults can make and recover from mistakes without ridiculous guilt trips.

If a fantasy teenager make a mistake, it seems that only the most extreme reactions can be permitted to her. Either it is all her fault, and most of the story will be spent in recovering from the mistake, or it wasn’t really a mistake at all, and the people yelling at her for it are being unfair. (Sometimes the author manages both at once, and then, of course, the teenager’s guilt dissolves like morning mist, leaving no scars behind). I think the second route is more common, since some authors seem terrified that a stumble will make their character unlovable—the plaster saint problem again.
Adults, though, benefit here from the fantasy author tendency to regard them as more settled and less dramatic. They still get upset and suffer extreme reactions, but they have longer lives to see the consequences of mistakes and how those mistakes aren’t always the end of the world—and to have made many of them in the past, though the author might only hint at them. Adults tend to be “imperfect,” in the good sense of the word, in the author’s eyes. So she lets them be human.
In one way, this is extremely sad, that the author can’t let characters who have little life experience and have a bad case of Terminal Innocence act human. But on the other hand, it does offer a huge advantage to the fantasy author who works with adult characters. All the best, most flawed, most real characters I know in fantasy are adults.

7) Adult characters can be responsible for others, playing more unusual roles.

This goes back to point 4, but that’s more about responsibility for themselves. Teenagers run away into the pounding rain while wearing silk clothes. However, they also don’t seem to care for others. If they take care of their younger siblings, those siblings will almost certainly get kidnapped, through no fault of the teenager’s own, leading to an exciting quest and freeing them from caring for others in the meantime. Most teenagers aren’t married, or are running away from loveless arranged marriages if betrothed, which leaves them open for adventures that result in their meeting their one true love. They don’t like their parents, or they’re orphans, which severs that bond. The author usually just starts from scratch.
Adults have it differently. If they’ve stayed in the same place for thirty years, they certainly have ties there, perhaps even, don’t die from the shock now, elderly parents, cousins, spouses, children, and neighbors that they like. If they’ve traveled, they’ll be known, or they might have doughty traveling companions to whom they’re attached. They get more roles than teenagers do. They can be mentors, parents, partners, spouses, lovers, former lovers, loyal and true friends, well-known warriors, rulers, full-fledged mages, nodding acquaintances, bitter enemies, and on and on and on. They’re more connected.
Connections can build just as many stories as starting over from scratch. Try it sometime.
Non-linear format rant is next.