This was actually what I was thinking, but sort of the other way around. We already have an age limit drinking, narcotics are more or less banned. I don't know

This was actually what I was thinking, but sort of the other way around. We already have an age limit drinking, narcotics are more or less banned. I don't know how the US handles this, but online gambling is a pretty good model for this. You need to be of a certain age, verified with a government ID (here in Denmark at least) and if you have a gambling problem, you can add yourself to a list the ensures that you're blocked on all gambling sites (and casinos).