He was 10 years old as I heard in the first season...
Judai Yuki said:13 Max.In the third movie he was 11.
Nathan Madien said:As of the Battle Frontier, I put Ash's age at 13-14. The reason is that it takes a long time to walk all over Kanto part 1, the Orange Islands, Johto, Hoenn, and Kanto part 2.
JJAB91 said:Not really I would give it more like a month or 2.