I think proof that Ash is at least 15 is that Gary is the same age as Ash, and Gary is now a pokemon professor. Since when are 10 year olds pokemon professors?
xD Since when are 15 year olds Professors? Youngest you see in maybe in their 20s! (Then again, those are GOOD professors.)
Ignoring Pokemon Logic concerning children and dangerous chemicals/pokemon;
I agree with 15 years old or around there. Each region is roughly the same size (I don't remember any game being discernibly longer than the other excluding Metal) and it took about a year for him to go through Kanto.
(I remember someone really saying "a year ago" or so around the league, anyone know from where?)
With 4 regions you get 14.
Add in all the other locations (Orange, Frontier) you get 15 or older.