The thing about age verification ia that it is at its core identity verification. You need to make sure the person on the other side of the screen is indeed who they are.
This means it isn't enough to verify the age of an account once. If all you need are verified accounts you just created a black market for verfied accounts.
That means you need to verify the identity of the actual person sitting in front of the screen with regularity. And that is problematic in various ways.