21 years old is already a legitimate age to gamble, and I would not call you kids in that age, with 21 years old you are already an adult ready to face the world and matured enough to make the right decision in life including in gambling.
Age will not be a concern with the respect to gambling in my opinion as I have seen even 30+ old men are gambling like a kid and facing very big losses. If a kid is enough mature in handling money, then allowing him to gamble (under supervise of parents) will not be a problem in my personal opinion.
But without having any idea on money management and not knowing about value of money, entering into gambling must be very dangerous. That might lead into losing all of their wealth at the end of the gambling activities.