This had been the main issue in online gambling. Minors had been flocking to the online Casino site without any restrictions. They can blatantly access gambling sites with their parents' cards, fake their identities and so on. Germany does tackle this issue seriously and recently German Commission for the Protection of Minor in the Media (KJM). had a deal with Incode Technologies, a US-based specialist in biometric identity for the age verification solution.[1]
The site itself will govern a strict registration platform that requires KYC if they wanted to strongly imposed NO Minors allowed. In this case, this will give no way for them to get involved in gambling.
But because many gambling sites don't require this (and to protect privacy), there is no way to stop them. Casinos can be possible but if we talk about online gambling that is very easy to manipulate and even use other identities. This is hard to regulate as people and these minors will certainly find a way no matter how hard it was.