I like the idea of keeping Spidey a teenager for a while. I think fans of the more recent "Ultimate Spider-man" series would also appreciate that and I'm a longtime Spidey fan who enjoyed the re-imagining of Spidey in the Ultimate comic-line (before it went to crap with the 'Ultimatum' storyline). Actually, the MCU adopts A LOT from the pre-Ultimatum Ultimate Marvel comics.
Also, when standing next to the Avengers, those are the heroes Spidey idolizes so it'll be an interesting dynamic between the teenage Spider-man and the adult Avengers...especially when they learn he's a "kid".
I think Marvel is also looking to approach this as stretching out Spider-man's stories and that eventually they'll do a movie where he goes to college and "moves on" to adulthood. Probably also why they're also looking for a younger actor to fill the role, so he can "grow up" with it kind of like Radcliffe did with Harry Potter.