Yeah, the brain of the kids is still being developed, so they need some time.
In general it is a good idea to have checks and balances that don't allow kids to be able to be around those things.
But at the end of the day, parents should teach the kids to be able to confront those things, because they will have to do that anyway later on.
That is a good point. If underages should not gamble because they are not ready for consequences and inexperienced, then the same +18 years old person or any older is similarly inexperienced and not ready for gambling. Because where would such person train and gain knowledge, if parents did not give him at least an introductory to gambling, a minimum information? There isnt a special switch in a person, which parent turn into ON position when their kids turn +18 and become fully responsible adults. Also I think it will be hard for parents to teach anything their children when they are +/- 18, because at that age kids think that «they already know it or know it better than adults». Try to come to a 18 years old person and try to teach him something. He will answer that he is an adult and dont need to taught anything.