You have to be of a certain age to be allowed to register at a casino. That minimum age depends on the country where you are when you play.
If you want to play for money in an online casino, you have to register. To sign up, a minimum age is applied, which is dependent of the country you are in. The policy from this country is directive, not your nationality. This means that in some countries the minimum age to be allowed to gamble is 21 years, in others, it's 18.