Even if perfect age verification is implemented, kids will be harmed. Because they are vulnerable while above the arbitrary age limit. So -arguing for age verification is harming children. Won't somebody think of the children?
And the answer comes back to the same point. Parents should mediate this. Online content should advertise its maturity level, and parents should be able to configure devices at setup time for the maturity level they are appropriate for. Then apps should respect that and everyone can be better off without robbing everybody of anonymity or freedom.
Other still need protection at 30, or in fact in my opinion also a lot of people above 50 (that didn't grow up with the internet around) are not prepared for what is out there. Also there are parents that are "underage". Those are not considered either.
I agree it is a very arbitrary limit indeed, but it seems there is no other good proxy so for a lot of people it will be choosing the best worst option, at least that is how the argument in favor is brought forward.