Engagement-maximizing has a whole bunch of problems. It creates a nonconsensual, exploitative feedback loop. It's bad for kids; it's also bad for adults. It does not appear to be meaningfully *differently* bad for kids vs. adults.
It is justifiable to age-restrict things that kids are A) especially vulnerable to (cars) or B) developmentally affected by (alcohol & drugs).
Neither seems to be true for "social media". (Source, vaguely: If Books Could Kill's episode on The Anxious Generation)