The Age Gap

I've noticed a little something during my adventures in TVLand lately. I don't know if any of you are enlightened on the concept of Dawson Casting-actors much older than their characters, such as a 27-year-old Paul Wesley as a(sort of)seventeen Stefan Salvatore-but I'm beginning to wonder about another variant of thing syndrome of inaccuracy. Is there such a thing as Dawson Writing?
(moderate spoilers for BtVS season 5)
Most writers are, in fact, older than their characters. And most of them do a great job portraying the oddments and troubles of the various ages, commonly the ever-popular teen years. One of my absolute favorites in this regard is iconic action show Buffy the Vampire Slayer. Joss Whedon does great-the whole show is really about growing up, and the weird experiences, both parodied and played straight, that come with it.
Funnily enough, I also consider Buffy to be one of the biggest offenders.
See, having a huge age gap between you and a character you're writing can be kind of...challenging. I would know; I've done it. Whether or not it was successful, I know not. But I do know something about the fifth and sixth seasons-that they were unsuccessful.
I don't know how acquainted any of you are with the character Dawn Summers, but I can tell you a few things about her. First and foremost, she's a good example of how not to write a little sister.
I hate stereotypes. You might even say I loathe it. And, as a little sister, I especially hate the "bratty younger sibling" cliche.
Dawn was fifteen at time of introduction, and, to be honest, I think we all hated her right off. There's a pretty good reason. Whoever wrote all her dialogue had decided she was five, not fifteen.
When it comes to teenagers, especially ones involved in crazy worldwide danger, I prefer writers who overestimate our intelligence. You'd be surprised at the stuff some teenagers have got up there.
Now, Dawn was lucky enough to have a halfway decent actress and possibly some character development. But not all are so lucky.
Think about this. How many characters have you met that seemed...off? Could have been because the mental state of whatever age they were was drastically misestimated.
If I'm being fully honest here, I'm not the best to criticize. In the words of Chuck Shurley, "Writing is hard." Especially characterization. Still, I'm an amateur. Sometime I want to go up to a professional and beg them to look a little harder. And that's basically all I've got for you.
May your swords stay sharp and your chocolate stocks always plentiful.