Like US liquor laws, it ultimately affects the privacy of everyone - I still have to give my birthdate to buy alcohol, despite my grey hair. Of course, I just make something up, but at strict places, they may ask for ID and so the cashier at the least knows.
Similar issue here.
On the other hand, it's pretty clear that most people under 16 should not be on social media. It's also clear that most people over 16 should not be on social media. Again, one could make a parallel with alcohol in the US.
