For fiat-based online casinos, you can't. It's part of their regulation. To somehow bypass it, customers used other identities such as families, friends etc. of legal ages.

For crypto, most gambling sites don't require KYC. Users can freely register to the site as long as, of course, if the country is supported or a country wherein gambling sites/activities are allowed.

And honestly, you can verify it to yourself if you will try it.