physical casinos will require an ID for anyone suspected to be a minor before they can enter the premise, not before placing bets. They should never be allowed to be inside a casino in the first place, so right in front of the entrance they will already be filtered out.
As for the online casinos, there are a lot of different ways the minors can by-pass that policy. So, there's not much an online casino can do with these regards. The least thing they can do is to impose the KYC verification upon registration, but that only discourage a potential client.
I don't know your country but physical casinos in my country don't require national ID to access it. Operators depend on physical evaluation to screen gamble who they assume is underaged. Such people might be required to present an ID or birth certificate if they are suspected to be underage. But some of these teenagers outgrow their age that you will not know that they are less than the gambling age. However, online gambling platforms are less restrictive since another person's KYC could be used by an underage gambler to access betting platforms.