How old is Finn in Adventure Time? Because it never says, yet he always looks the same from time to time when surely they can't have trouble every day.
17 years old, at time of posting this answer
In the Season 6 finale "The Comet", Finn states that he is 16:
FINN: Besides, it was like 40 years ago. I'm 16! I don't have a star to revolve around to track time.
Since time in Adventure Time elapses at the same rate as in the real world, and since we are currently near the end of Season 7, this would make Finn 17 years old.