I once heard Terry Pratchett (at Hay, I think) muse on how odd it was that shops purporting to sell "adult books" invariably sold what could only be read with pleasure by a backward 14-year-old...

I suppose it's yet another example of the way in which it's the people closest to the border (in this case of childhood) who are the most insistent on its maintenance. Spotty youths are more generally more anxious about being mistaken for children than machewerer individuals and embrace the Adult label in a tighter clinch. Maybe.