Only really honest platforms will stop underage gamblers from playing if they can identify them but majority would not bother to check.
I think you are right, I have noticed that there are many teenagers who are into the gambling thing and yet no casinos care about their age because they don't need such details of verification for short amounts of withdrawals. Well, I would also say that those casinos that ask for KYC when your withdrawal amount is higher aren't always shady ones, because they ask for KYC to better know about their customers. Once the KYC process is done they will allow the user to withdraw the earnings without any issue. They have been doing this for a long time and have built up respect in reputation in this community.
I'm quite sure that most of the reputed casinos are honest and they accept those users for gambling who are in acceptable age group for gambling activities. I'm not sure about gambling casinos, but there are many applications that allow teenagers and even the children to play games like poker. I know that there isn't real money involved in those games, but still morally it is unacceptable by many people around the world.
Sorry, but I don't agree with you, because it is far easier for minors to do gambling at online casinos then the physical ones. The online casinos really don't check each user's age even if the user fakes it, because they work mainly on the model of crypto-currencies, and they mostly allow everyone to freely enjoy the platform without any restrictions, but I do agree that they try to not allow minors on their platforms. However, they can't monitor each user, and that's also a fact.
On the other hand if a physical casino is allowing minors to play the casino games even they can see that the user is below the age of 18 then they are doing it willingly. They allow the minors to play the games for the time when they are losing and when they start winning then they could create troubles for the user by asking for his age proof to show that he's at an acceptable gambling age. I think those shady casinos don't really care about someone's age, they mainly care about the money and that's a fact.