around 7-8 for pokemon tcg, 6 if they are very advanced.|
You can buy a few packs of cards or a bunch of crappy commons on auction sites though and give him to them now to start getting him interested. Play without energy and use the numbers on the attacks and some gem style counters (or pennies, cubes, etc.) to show the basic battle mechanic. it will help him/her with math skills. Plus you can make funny sounds and just pretend your PokieManz are battling.
This is about how my son plays. We don't even have full decks -- he has about 40 cards. And the grownups he plays with just gamely go along with it.