As the parent of a teenager affected by this ban (plus one who has aged past it): I wish that it had been in place 8-10 years ago, before either of my kids got smartphones. We tried to be reasonably conservative in their introduction to devices and social media, on the rationale that it would do them no harm to delay using those for a couple of years through their early brain development. The real difficulty turned out to be the network effect of their peers having access to social media, which increased the social pressure (and corresponding social exclusion) to be online. Not having access to Snapchat/Discord/etc. at that point meant that they were effectively out-group, which is a Big Deal for a teenager.
We ended up allowing them onto social media platforms earlier than we'd have liked but imposed other controls (time and space restrictions, an expectation of parental audits, etc.) These controls were imperfect, and the usual issues occurred. My assessment is that it was a net negative for the mental health of one child and neutral for the other.
I realise that HN is primarily a US forum and skews small-government and free-speech-absolutist. I'm not interested in getting in a debate with anyone about this - my view is that most social media is a net negative with a disproportionate harm to the mental health of non-fully-developed teenage brains. This represents a powerful collective-action failure that is unrealistic to expect individuals to manage, so it's up to government to step in. All boundaries are arbitrary, so the age of 16 (plus this set of apps) seems like a reasonable set of restrictions to me. I am unmoved by the various "slippery slope" arguments I've read here: all rules are mutable, and if we see a problem/overreach later - we'll deal with it in the same way, by consensus and change.