It won't. It will actually take away all responsibility for SM companies and this is one of the biggest criticisms, they will now be absolved from a lot of responsibility as 'under age kids shouldn't be on it'.
It is much better to educate children around being safe online than ban it til 16 and then a free for all.
This hasn't worked in the likes of Australia. Kids have already bypassed it. Its a waste of resources. Education around cyber bullying and online safety would be much more beneficial.