No, that would be a form of theft. Even if you are underage, it is their fault for not id'ing you before you played. It is their liability, not yours. Even if it was your liability, they still can't steal your money. The only action against you if it came to that, which is unlikely, would be on a separate count such as a penalty/fine for playing underage. Winning money is completely separate. What if you lost? Are you able to recoup your losses cause you were underage? Of course not. Works both ways. Sure, they might try to intimidate you whether purposefully or accidentally to get your money especially if you have large sums of money, but this will never hold up in court, unless you're on tribal land perhaps