Those are probably a reflection of legal issues. 18 year olds are clear from child protection laws, and 21 year olds can be shown ads for alcohol (and maybe tobacco, I think that's 21 in many states now).
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.
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.