from what i know, it is 21... when my brother turned 18 we had gone to las vegas for my mom's business meeting, so it was my stepdad, me, and my bro in the airport (my mom had gone a few days before). He decided to play a slot machine there, won $5 and got printed a ticket to go to the cashier. The cashier asked him how old he was, he said 18, and the cashier said it was illegal for him to be gambling. He went ahead and gave my bro the $5 and said to just not do it again.