Plenty of stuff out there is not appropriate for kids. Securing network via filtration is pointless on a mobile device, and kids are more than smart enough to get around various other mechanisms to access things they shouldn't be able to, including social media.
The principle has always been that something that could be secured in real life (18+ shops and events) should also be able to be secured in the cyber world, but there was never a practical way to validate it that didn't also risk exposing the token and identity of the user access it to be vulnerable to exposure. Well, we have a system now. Smart phones, like them or not, function as de-facto IDs already, and there is no rational reason for Google and Apple to be unwilling to associate the age of the cloud account holder as an AVS somehow. We have the biometrics, we have the pervasive internet, we have the standardized protocols and simple QR-encoding of relevant authentication. Banning under 16 users (for example) from Snapchat is a single, simple switch that the industry has not been willing to enable. If legislation is required to force their hand, so be it.