I believe that the gambling site need not to verify whether you are up to the age of 18 or not but they expected you not to lie about your age and do the right thing by clicking yes are 18 and above or no you are under age. Once you click yes that you are 18 and above while you are actually under age then whatever happen to you in the process they are not responsible but yourself.
Yes, because gambling is an adult thing to do. That's what the majority of them expect but there are also those circumstances that a teenager would sign up to a casino since it's on the internet and all of the casinos are accessible as long as you're not from the country that they've blocked their domain. It's really hard to figure out if a minor has already signed up.
IIRC, there's the case of this teen that has used his parent's information to be kept in that casino since there's no need for any face-to-face KYC interview. It's just all about sending an ID and filling up the form.