I've commonly seen this cited as a benefit of uncircumventable age gates, but I think that most proposed systems I've seen are far stricter than they need to be for this: they have a hard line exactly at 18 or 21, and allow neither read nor write access to gated-off sites. Should a 17-year-old teenager be able to access no more social media than a 7-year-old child can? And must read access always be gated, even though gating write access would be sufficient to prevent this overrunning? Teenagers have to learn how to operate in the adult world at some point, and I worry that pushing so much access to age 18 all at once could have major unintended consequences.
As long as there is potential, significant liability attached to showing under-18's certain content it's safer to just not show them anything questionable until they're 18.
I'm with you, I think a gradient would be developmentally better, but our legal system discourages that. I think the same thing exists across other parallels; people get a ton of options dumped on them at 18. It would probably be better to phase that in over time.