I think there are a lot of people gambling under the legal age. I believe that it's going to be hard to control that access online gambling sites just because when using bitcoin as the currency, there are no verifications needed.
yes there is no doubt about that fact that there are a lot teenagers who are playing gambling and specially though online gambling, where they do not have any restriction, so the number of teen age gambler will be more in online world.
There's no rule about the age limit on gambling sites so teenagers are free to gamble in any online casino they wish to play. And even in real life there's a lot of teenagers who learn gambling at a very young stage. They sports bet with their friends so that's another way for them to gamble while they are young.