This is why gambling monitoring of KYC verification must be strictly imposed to avoid this. Minors shouldn't be allowed in gambling platforms because their prefrontal cortex which mainly affects their decision making isn't even fully developed yet until the age 25 according to studies. They can't make rational and logical decisions just yet. Additionally, most of them don't even have the enough money to pay for their own bills and to survive on their own. They are still very much reliant and dependent to their parents. So technically, they shouldn't really be allowed to gamble. However, if strict KYC will be imposed, the gamblers who are particular with decentralization and anonymity will have to compromised.
I have read many times that children use their parent's documents to verify and get registered on online casinos. Then they use their credit card for deposits and eventually gamble, the parents would only learn about it if there has been a significant increase in their credit card bill. Eventually, the responsibility is on the parent to monitor the child's activities online. Which most parents are unable to do due to their hectic life, which has become common these days. Strict rules won't help unless parents monitor every activity of their child on the internet. Now comes the worst part if parents do keep a tab on every activity of their children they as the children question their parents about privacy. The woke system has made is so hards for parents to control their children with laws that do not make any sense in these times.
You cannot monitor everyone's activity in an online casino unless it concerns the casino itself. For implementing strict monitoring protocols to determine the age of a user these online casinos would require to invest heavily in their verification infrastructure, they would need humans who would need to verify every document manually. I do not think it would be practical for any online casino to implement such an infrastructure when they are neither in profit nor in loss. A casino be it online or offline objective is to make a profit and lure new clients. If they are not able to do that then it is better to close the business.