Well in the modern society today once a kid gets to 18 years, they will most likely consider him an adult but the truth is that even some persons at the age of 18 aren't still mature enough to make proper decisions for themselves. 18 is more like a general average, some kids around 16 are more matured in decision making than some persons over 18. Sometimes kids that are pushed into gambling is as a result of unnecessary peer pressure on them and that's the more reason parents need to be close to their kids.
Personally I think gambling even at the age of 18 is too early and that's because younger people tend to misuse opportunities wrongly compared to their older counterparts.
Perhaps that could be the reason some people only see age as just a number and nothing else because maturity comes from the mind through their behavior and their decision making because we always see some people who are older but still act and think like an under age children while their are also under age just like you explained that are very smart in decision making like an adult, though sometimes up bringing of children can affect them negatively or positively because some believe that children who are expose from early age to source for themselves tend to understand things better than those who have everything at their disposal, so even if 18 years is allowed for gambling but those that cannot make a better decision for themselves should not be allowed until they prove maturity in them.