I don't know the reasons of the other country why they allow below 18 age to play gambling and bad with that is they have child they can play. I hope other country will follow the age limit of minimum of 18 to play gambling because that is the right age for us . Even they have law that still not follow because many children play gambling.
In the Philippines many teenager play gambling and bad with that is they play outside it is very sad to see those children instead of studying they do playing and mostly also of that are addicted to the drugs.
The reason why they allow below 18 to play gambling is that it is legal with their law.
One more thing is that underage was allowed to play depends on the house rules of the house itself that even the law allows it.
In that case, their parents are the one who's responsible for their actions, and as far as I know, if those teenagers will be seen by the law enforcer, they will be punished.