How old is the under 18 exactly?
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.
If we are talking about under 18 years gambling online, it does not depend on the country where the OP resided but according to the rules and regulations of the online license Master holder which granted license to an online gambling site per say and the last time I checked under 18 years are all restricted.