From my understanding I don't think there's any law that restricts an individual that's 18+ from gambling, that's an adult age and in must countries individual that's fall in that category are allowed to make their own choices, but I heard that their are some countries where the gambling age is 21 and above an not 18 which is the adult age though I'm not very sure about that statement, but in general the gambling age for most casinos and betting sites is 18+
As a parent it's your duty to guild your child till they become adults and start making their own decisions however they still need advise on certain decisions that could ruin them if not properly looked into and gambling is one of them, cause it has lots of disadvantage as well, like addiction, emotional trauma and the rest of them. But with proper advise to them, they could learn from it and try to make the right decisions.
Maybe in each countries have the difference of allowing someone to playing gambling, including what age they can playing gambling. That will be different from one to another country but if we see what is written on the casino, the allowance of people to playing gambling is above 18+ so they who is under 18 can't playing gambling. Although they can still playing gambling without anyone knows, especially if the casino doesn't have strictly rules. That will gives an easiness for people who under 18 to playing gambling and they will not have a problem until the casino asks them to fills verification.
Yes, parents jobs is guides their children not to make a wrong decision, especially if parents found their children playing gambling in their room. Parents must explains what impacts gambling to their lives and what will happens to them in the future. Parents can gives many examples to their children just to show how dangerous gambling for them, especially if their children doesn't have self control.