User blog comment:Adventuretimegurl123/OH MY!! HUGE ADVENTURE TIME NEWS!! :D/@comment-5133095-20130514184104/@comment-84.29.25.8-20130529211938

He ages, like everyone else. They don't really show it, but he does grow up. Also if he would still be twelve they would have had to find another voice actor for Finn. because when you compare a season 1 episode with a season 5 episode you can definitley hear a difference.

So there, your evidence is common sense altough I don't know whether he'll be 16. He turned 1on the 13th of March 2011, so when they start airing season 6, maybe in 2014 or late 2013 He will most likely be 16. So yeah, there's your proof.