I'd argue that most things that are currently in the crosshairs for exclusion under age verification are also harmful to at least a third of the adult population and to society in general.
Actually maybe that's just for profit algorithm based social media and / or mass scale surveillance and personal information gathering and advertising.
The point being, if you're going to make a case for something being harmful to kids, you need to also make a case for it's being OK for adults or maybe it just needs banning outright for the good of society, see also smoking. Personally I'm in favor of leaving this in the hands of the individual and parents, and perhaps making easy tools for less technically adept parents to use.
TLDR: If Facebook is bad for kids, why isn't it bad for adults?