Anyone who can control themselves can as well gambling and of course gambling is for adults which is from 18+ and above, anyone who is above this age bracket can gamble provided they can gamble responsibly doesn't affect them. What matters is, stop when they should stops and exits when they should do that instead of continuing gambling and Harming themselves by continuing to gamble knowing too well that winning isn't fort coming yet decided to be chasing after what they had already lost whereby causing problems for themselves and the people around their environment.
The focus is indeed on those who are of legal age for gambling, but ultimately, we can see that with the rise of gambling, including online gambling, the notion of gambling being only for those of legal age is no longer valid, as gambling can be easily accessed by anyone, regardless of gender or age, and it is not uncommon for teenagers or those still in school to gamble.
In my current environment, there are even some middle school aged children who are still far too young to gamble, but they have already been exposed to it, which clearly raises significant concerns.
I don't think it's a problem if adults gamble, but children who are not yet ready to know about such things can clearly be disturbed in their thinking and psychology if it is not stopped.