Interesting question.
One thing that pops to my mind is that the "old day" classics for boys were very often about guys in their twenties or late teens. Three Mousketeers, Robin Hood, etc. Young dudes in prime time for action and romance.
Nowadays it seems that the young are almost being force-fed "same age heroes". This may be natural, or it may be an industry machination like word lists and all the PC-friendly crap you see around the library with empty checkout cards inside.
Hard to say. I would say that kids tend to emulate those slightly older and disdain those slightly younger.
Since I'm currently finishing up a contemporary SF novel in which almost all the main characters are teenagers... but is apparently to rough to get an official publisher YA nod... this is something I've been looking at a bit.
This might be peripheral to the question here, but I've pretty much decided that the book will be pitched as an adult novel, but I will heavily promote it to teenagers...many of whom are all over the very sex, violence, and dope stuff that the YA publishers apparently avoid.
So I looked around for: how many adult novels feature mostly younger characters. (In other words: Exactly how screwed am I?)
What I came up with was, they are out there. Dune, for example. Catcher in the Rye. Count Zero. Mona Lisa Overdrive. Harry Potter, for that matter.
I think I'm okay. And ideally, since these characters will progress into middle age over four books, I can cover all the bases at once. Mostly I want kids who have outgrown Potter and like Grand Theft Auto, Matrix, Fight Club, and "urban fantasy" if I understand what the hell that is.