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.
I was enjoying the defence to the point that they can still be told to bring their ID or birth certificates if they are suspected to be underage by physical casinos, heck no in my country, that ain't happening. You are free to do whatever you want to do at the physical casino, you may even lie that someone sent you.
The operators here are not even trained to that extent, it's all about the money for the casino companies and the government doesn't even look at that angle, do we even have one? It got worse that an adult could send a young child to the casino to gamble a prediction written on a sheet of paper for him/her. This is wrong but it is what it is.