Cannot Work Without ID
The only way this proposal will work if everyone is required to use 'official' ID in order to use social media, as everyone will need to prove they are over 16.
I also think 16 is too old. I think 10 or 13 would be better ages to work around.
My solution would be to make it a legal requirement that any child's social media account is actually controlled by an adult via a system similar to the way Google handles children's accounts with family options. It would be down to the parents to decide how much or little the 'child' can do on their accounts but the parent would always be able to view 100% of the information on there.
So when a child reaches 10(13) they can have an account if the parent allows it, but the parent has full access/control. They can determine how much freedom they have when using it.