18 is the most common age that is considered matured enough even in drinking alcoholic beverages. I think same should be done with gambling. But, I think gambling sites don't really have the control over their customers or players. For sure, those with age lower than 18 can still play if they want to and just fake the information, particularly their age, in the KYC if there is one.

It's still the responsibility of the parents to know what their kid is up to. Disciplining the child while it's early is necessary so that when their child grows up, he would know what is right and wrong and wouldn't be a pain in his parents' ass.