Under age gambling is what both online and offline casinos don't joke with and some times why the government are so strict on gambling licensing os because of the KYC implementation which will help to expose any possible under-age registration and from the document verification you will be sure to be able to cash up with any of such possibility of underage.
Also, this is a hard battle for them because they won't determine if the verifications and IDs sent to them is the actual owner and that's why they are being stricter with such policies as for holding the ids and selfies. Because kids can just ask their parents to do it if they have the consent but as long as they have their own measures in doing it, there's no problem at all and they are only complying to the rules of the government that's being placed on them.
The cosino have the sole right to request for verification from the gambler anytime they feels that the gambler may be an under age and since government laws already cover that aspect is much essential for casinos to comply with that law at all time to avoid sanctioning.
Yeah, if they feel that there's something wrong on that account. They're free to do it as that's their own platform and they're the kings of it. Asking for additional verifications and IDs is typically happening in every casino that we know of.