The contention is that the thing in question is harmful for minors and adults, albeit perhaps to different degrees. Also, to be clear, any ban should be enforced on the offering side, not the consumption side.

You can easily argue that most of the things that are banned for minors but not for adults are also harmful or at least dangerous for adults as well. Alcohol, pot, tobacco, pornography, stripping or acting in pornography, gun purchases, etc. are all debatable as far as adults, but clearly should be out of the question for developing brains. Perhaps an even better parallel to social media is that minors cannot get credit cards or take out loans without parental approval. A social media profile is a bit like taking out a mortgage on the rest of your life.

There are things that can have lifelong harmful consequences that we as a society recognize adults have rights to, and which they may be capable of moderating their exposure to, but which minors are simply not prepared to fully understand the consequences of.

Banning minors from social media does not ban their speech or access to speech. It bans their access to the gamified drug-like patterns of engagement surrounding the commoditication of speech for the gain of companies which know full well that the services they provide are built on hooking someone's eyeballs at the earliest age possible.