I don't think they really need to.

Laws are often just an acknowledgement of a society's expectation.

"We've all decided that kids under 15 using social isn't great."

The fact that this law exists makes it infinitely b easier for parents to establish and maintain rules in their household, because peer pressure is minimised.

Yes, some kids will still use social before they're 15. Perhaps most kids. However, I think harmfully excessive use will be minimised.