Currently to be an online casino player there is a pre-defined and may well be a player who has provided the notion of what you are doing. However it is impossible to be a professional player is not of legal age, since it is one of legal rules to play for real money in online casinos. For just like playing with play money to improve his technique and his knowledge, there is no minimum age, because knowledge is never enough, but to play for real majority is generally present in an all online casinos.