It’s physical so easier to enforce at point of sale, during school, at the mall, etc. eventually the social pressure will hopefully subside so home enforcement becomes a non-issue.
I admit to not yet living through the challenge of having my kids exposed to "social media" (aged 9 and 5, no phones yet, the older one does have a laptop with Linux on it), but I feel like we are trying to punt as much responsibility on the government that we as parents are failing at.
The analogy with things like smoking does not really work: social media can be used to educate, to connect and even bridge divides. Yes, it can also be used to harm, but so can a book (its content, not just physically).
If there are specific concerns (eg. adults messing up with kids? videos/photos without consent? shaming of others?), let's make those clearly illegal, and possibly have a parent/guardian vet the social circle.
Otherwise, we don't teach and let the kids learn to selectively digest social media content, and instead they get thrown at it at age 16. But let's admit it, adults are just a bit less pliable to influence than kids.
But my core point is that a government can't be solely responsible for the upbringing of our kids.
As I said, my kid already has access to the internet through his laptop. I don't particularly micromanage his access, but I do talk to him about things like online gaming communities and such. I run a Minecraft server for him to play games with his friends too.
Obviously, asking every parent to go to these lengths won't work, so ultimately, I need to ensure he is making good decisions, and I need to do that along the way. Do I need government support? Yes, they need to react to reports of shitty behaviour, and schools and kids communities need to be strict about the worst stuff kids tend to do before they develop sufficient empathy. But banning outright I don't want them to do, because I need to be able to support my kids through these experiences during the ages they are less likely to be rebellious and contrarian (so, I guess before they turn 12-14 :)).
Not that I agree with the approach (digital IDs, age verification, etc), but of course a social media CEO doesn't want to see it's user base restricted.
I believe a more apt analogy is with books. There are books we don't want kids to be reading too early, and while there is a higher barrier to entry (enough money to print and publish), we never consider banning all books for under 13s or under 16s!
Let's make harmful content illegal or restricted, but let's also take responsibility as parents to do, well, the parenting.