21 and over can legally play for a cash prize in most states IF its a game of skill...which pokemon is. It's much like bowling leagues.
In Washington state Pokemon would be considered game of chance, thus if there was an entry fee involved (30+3 for example) it would be illegal gambling.
The law here says that if there is ANY element of chance then it is a game of chance. We have been tryng to get poker recognized as a game of skill for years to no avail.