I don't think it dilutes it. If anything it should reinforce that sharing any material under the age of 18 is a no no.
I agree that in some cases a 19 year old isn't fully developed, but in that case perhaps the age should be raised to 21 for sharing material rather than disregard a younger age because of it.
I think it should be whether or not pedophilia/hebephilia is involved or not because that's what the law is intended to fight and why the punishments are so severe. In this case, I think it has nothing to do with it. In all likelihood, 5 years from now, 16 year olds will be children to him (probably are already) and he won't feel sexually attracted by them. What should be reinforced in this case is that sharing any material without permission should be a no no.