I don't see any problems for kids to play poker at home with their friends for small money, nickels and dimes, where nobody can get hurt or lose anything significant. I use to play a game called 31 with my grandma for nickels and dimes and if I won, I got to keep the money.
I used to play poker with my cousins when we were pretty young (like 8 or 9), we actually played for free (bragging rights), because when your a kid, poker is still fun to play for free
As far as online poker for real money. I think kids should really be at least 18. It helps if they have a job, and have an uderstanding of what money is, and the fact it doesn't grow on trees (or their parents wallets).
Jason