The age limit placed on people below 18 year to keep them away frim gambling have not been most effective as we have hard records of under age gambling in many place and this is a pointer that casino has indeed misolaced their primary responsibility which is to prevent that from happening and that is why there is need for KYC.
The underage of 18 years old is a part of the reasons why KYC is compulsory on most online casinos but not all online casinos requires KYC verification, there is freebitco and others like this platform that underage kids can gamble on. They can also passed the KYC if they can get a hold of their elder brothers ID or even their parents, unless the online casino has the latest KYC style that ask for your selfie live.
Unless the casino will do that for sure it serves the purpose of KYC, but like what you mentioned it can easily be hack by minors especially
those who knows how to play with the system.
They can easily submit other's ID's and pretend to be them and continue enjoying playing, with the knowledge that they can acquire online.
It's very possible that those young folks can work it out even there's already a KYC requirement.
I undergo KYC on various reputable casino here in the forum and all of them require selfie verification that holds the ID and piece of paper written the current date and casino name. I doubt that a minor can dodge this kind of selfie verification unless if they will use fake ID and make themselves to looks like old enoigh in the selfie photo.
Exchange also have this kind of verification process. I think a minor can only play on decentralised casino but not on centralized casino with strict compliance with their license provider.