Mixed content isn't a problem for broadcast TV or newspapers. They simply avoid publishing material not appropriate for children, e.g. broadcast news programs don't show gore, even when a place has been bombed and is littered with body parts.
Think about the Twitter home page (back when it was Twitter). If someone tweeted something that should be restricted, what do you suggest?
Mark the whole page over-18 and effectively ban kids from using Twitter until it falls off the front page? That's ridiculous.
Ban people from tweeting over-18 content or hide it from everything except direct links? This is what Twitter management would choose, in reality, if the other option aa the above. They'd just make the whole platform child-safe. Remember how much outrage this caused when Tumblr tried it? But you want to force this on all platforms.
Hide the over-18 tweet from users who are under 18, but display the rest of the page? This is the obviously most sensible solution, and it requires the server to know whether the user is over 18.
X lets post authors mark their content as NSFW, and I'd assume they're doing some sort of algorithmic checking or making use of user reports of content to classify content too, to varying degrees of success. It's definitely subject to the whims of the platform owner, given the sheer amount of racism, sexism, and porn that isn't marked.
My account is >18 years old so it never showed up for me (it just used the account age to determine I'm old enough), so I'm going by what others have said.
There would obviously arise a market for a moderated, or more regulated, version of this type of social media.
Social media doesn't deserve an exemption from the efforts society makes to protect children in other domains.
Those that don't, or don't care, would not seek a rating, and would not be accessible to devices that parents control. Otherwise they would be free to host whatever legal content they wish.