There is already one - it’s called RTA (‘Restricted to Adults’) and is a meta tag that should filter out sites if you have parental controls enabled. I expect it’s one of the things Google etc. look at for SafeSearch to remove those results too (among other checks).

Of course, the irony is that only those that would label their sites as RTA already (so are already easily filtered) would comply with any ID requirement, so these kinds of laws achieve very little!

I mean, just talk to your kids about sex. You're not going to succeed in censoring their internet activity. Any more than you'll succeed in stopping them from sneaking out and going to parties or drinking with their friends - did your parents try to stop you from sneaking out? Did it work? Because I got really good at scaling fences and climbing out of windows.

But in the effort you will send the message that there is no trust between you and your child, and they won't feel safe talking to you should they need to. If you take the attitude that sex is something illicit, they will take the message that it's something to keep secret from you.