Yes, I'll admit this was another late-night post idea I wrote up after midnight.
I'm not quite sure what set off the chain of thoughts that created this post, but anyway, I started looking at my collection of fantasy novels (with some science fiction thrown in for good measure) and noticing some commonalities. Mostly on the age of the main characters.
Talia, in Arrows of the Queen by Mecedes Lackey is just thirteen when the story begins. Now, this is a book that is just as suited for a teen audience as it is for an adult one. In fact, the library has the books shelved in the Young Adults section, while the bookstore classes it as fantasy.
Thing is, this seems to be almost typical of fantasy novels. Granted, she's a bit younger than many of the others - I think a reasonable guess for age would be sixteen to eighteen years old for the characters in a lot of the other popular series and books
Nearly all of the main characters in Lackey's Valdemar books are about that age: Vanyel is in Magic's Pawn, Lavan from Brightly Burning seems to be more Talia's age. So is Darian from the Owl trilogy. And it's not just the Valdemar books. Vetch from the Sanctuary series starts out quite young too. As does the main character in the Bardic Voices series (Lark, I think, though I can't remember her actual name right now)
Other authors have done about the same thing. Harry Crewe from Robin McKinley's The Blue Sword, is eighteen if I remember correctly. However, that book is currently classed as Teen fiction, and is also a Newberry Honor book, rather than regular fantasy. So, it (and several of her other books) are something of an exception in the category.
Admittedly, as I've noted with The Blue Sword, these books could be marketed to a teen audience (or were intended for such an audience to begin with), but that's not always the case. Given some of the things Paksenarrion sees in Elizabeth Moon's The Deed Of Paksenarrion, there's no way I'd call this book safe for teens (at least not the younger ones). In particular, I'm referring to the scene in Liart's den. But, there are others too. So, this book is more clearly for an adult audience, and the character still starts out at about eighteen years old.
Similarly with Robert Jordan's Wheel of Time series. It's definitely been a long time since I've read any of that series (to be honest, I gave up with Winter's Heart), but I do remember the youth of the characters, at least at the start of The Eye Of The World. And, it's another series I don't consider to be specifically teen-friendly.
I'd suspect this trend has something to do with the pseudo-medieval setting a lot of the fantasy worlds are set in, but there's a fair amount of science fiction that does the same thing. Andre Norton's Solar Queen series starts out with the main character just out of training (about eighteen to twenty years old, I think). Some of the Darkover books by Marion Zimmer Bradley too. I'd give specifics, but the book in question is still packed from my move.
Not to mention Star Wars. If I remember my chronology right, Luke can't be more than ninteen or twenty years old. Now there's a set of movies that appeals to all ages!
Is it just a formula that the writers/publishers have found to work for selling books? I've read some very good science fiction and fantasy that stars older characters, so it's obviously not a requirement. Old Man's War comes to mind quite nicely for example. So, what is it about younger characters that make them so interesting to us? Or, do the publishers and authors think it's mostly younger readers (teens and folks in their twenties) who are interested in reading fantasy/science fiction?
I have to admit, these days I'm finding the older characters with life-experience to make stories that are a bit more interesting, such as The Adept series by Katherine Kurtz, or more recently John Scalzi's books.