https://www.polygon.com/2018/7/5/17536254/how-old-is-ash-ketchum-pokemon

Ash Ketchum began his Pokémon journey at the age of 10 years, 10 months and 10 days.

We know this because it’s stated in the beginning of his story, written by the Pokémon anime’s chief writer Takeshi Shudō. The rules of what is and isn’t canonical in the world of Pokémon may not always be black and white, but this seems like a safe detail to assume is “real.”