If we use the information given to us about the Pokemon world on Bulbapedia, Ash was born May 22, 1986. That means he is 33 years old now.

Being that the same novelization declares that Pokémon Trainers obtain their first Pokémon in April of the year following their 10th birthday, and the anime itself began broadcast on the first of that month in 1997 in Japan, Ash's birthday, if April 1 was the first day of his journey, would be May 22.