How old is the under 18 exactly?
If by "gambling" you mean "taking big risk on bets" then I can assure you that you that you are not supposed to gamble even as an adult. You can however bet little, and only what you can afford to lose. But betting as under 18, it really depends on the country you are from. Does your country allow kids to bet without supervision from guardians/parents? *Gambling* is out of the equation here because kids are not supposed to gamble at all.