To be honest, your question is very misleading or should I say confusing instead. When you said "to be matured enough to gamble", it's like you were saying that they are obligated to gamble when they reached the age you mentioned, so here's my opinion.

A person once he reached 18 or 21, he shouldn't have really playing gambling, I guess what you were trying to say is that he is allowed to be inform or educate what is gambling, so on and so forth. And that is when the right of freedom to choose will be claimed or do, whether he will play gambling out of curiosity or he won't cause testimony and real life stories about gamblers is enough for him to be scared and leave the gamblig world alone.