If the main intention is to prevent under-age players from participating in gambling, then I would say that KYC will be having hardly any impact. I have seen kids as young as 12 years using the Tor browser in my city. Don't you think that they will be tempted to use illegal gambling sites (where KYC is not required) in case the legal ones ask for KYC?
Who knows if these kids are really into that?
The minds of these kids today are hard to predict, they are up to something new although they are aware that it might harm themselves. There are legit sites that does not require KYC too.
When you say legit, do you mean legal? Or licensed? Please cite those "legit sites."