None of these things are a problem.

> 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.

> Adults can be prosecuted for helping minors circumvent the checks.

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.