I... er... that you... should look for it yourself?
Y'know, to tell you the truth, I may have recalled that 'tidbit' incorrectly.
I did a quick search on Google just now and found no evidence of Mister Ash Ketchum currently being 15 years of age in the Pokémon anime series. Rather, it seems as though all of the show's protagonists are locked in a state of perpetual youthfulness.
Er... I... uh... must've kinda imagined the part about Mister Ash Ketchum being 15.
I plead forgiveness, as I did not intend to mislead anyone. I realise now that making half-assed assumptions based on heresy found on Pokémon forums probably is not the best way to spread information.