I know that Dawn started out at the age of 10, and I'm pretty sure it was the same for Ash, but it's been years since Ash started out and he still looks around 10. So how old is he really?
I know the truth is daunting, but he is actually 10.
He is timeless to make the show last longer. Its weird; and you could get technical, and if you do, he's in his mid-20's.
Source: It was announced in the first episode in Unova Adventure.
Hope this helps! (>^-^<)
He might be 13 years as he had more than 5 adventure series