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.
Maybe the point is when people go to a gambling place that provides games physically provided traditionally so that to enter the place the guard will verify their age before they are allowed to enter and be polite in it. Even though this requirement is given there are times when they can get past the guard because they may see their body condition is much bigger for a minor usually.
For online platforms it is much easier to manipulate because even though they are asked for KYC they can use someone else's identity before trying to gamble and even KYC applies when they want to withdraw their winnings thus providing an opportunity for minors to play.