I do not like this argument. Does an adult theater let anyone in without an ID? Do movie theaters let kids under 18 into an R rated movie without ID? Why do we think this should be different on the internet?
IMHO, they should have Youtube 18+ (Adult book store) where you need an ID and YouTube <18 where you do not (Barnes and Nobles), and they can filter the content as apporpriate.
I think it is evil that YouTube enables kids access to both grooming and violent material.