> 15 year old has a 18 year old friend
Adults can be prosecuted for helping minors circumvent the checks.
> Or the fact that other 15 year old looks older than some 20 year olds
See Australian approach. Site can verify you and both government and site don't know who you are. No need for photo.
> shocked parents finding
No law is a replacement for bad parenting. But good parenting is easier with the right laws.
> a one-time over-18 age verification check
it can happen more than once non intrusively.
If you skip the need for privacy protection and just retain a copy of the photo ID used to verify the account? Then sure.
But if age is validated in a way that preserves privacy? Then there's no evidence linking the adult to the crime.
And once that adult committed a crime anyone involved can report it, including that minor friend. It's blackmail material. Better not fall out!
Over the next five years, you can look forward to a steady trickle of stories in the press about shocked parents finding that somehow their 15 year old passed a one-time over-18 age verification check.
The fact compliance is nigh-impossible to comply with is intentional - the law is designed that way, because the intent is to deliver a porn ban while sidestepping free speech objections.