Lol, I can imagine Ash being 20 years old. xD
They do this to keep his immature thoughts and all. Everyone loves Ash as a 10-year-old, and so Nintendo want to keep him the same age all the way through so they can get their money to keep themselves running.
Of course, there is the possibility that he is aging (for example, he might be 13) but it takes him three years in order to go through the regions he progressed through. I don't know, I think that's what it is. Either way, Nintendo's getting money to develop more games. Everyone's happy. :)