I agree with anyone who says never.
It's not about that age if you like Pokemon, it is that interest to it.
These people who says "Lol, you're 24 year old and still watch/play Pokemon", Oh please...If you like Pokemon, then you like it.
My Uncle I just figured out that he is 58 and he still plays pokémon. I got a shiny Bronzor from him xD. Also my cousin also plays and me and my sister, her and my uncle, had a double battle. It was fun. Oops. Sorry sis for fainting your Dustox.