I went to my first con when I was 14 and I did my first cosplay when I was 15. I'm 16 now and I still think the vast majority of congoers are older than me. Id guess the average age of people at cons would be between 20 and 25.
Anime Boston 2012 cosplays:
Reggie Rocket (Rocket Power)
Anime Boston attendee since 2009