Just going by memory, but I THINK in US history, the age of Majority has ranged from 25 at times, 21 at other times, 18 at other times, so it seems to fluctuate in different historical periods.
There’s a number of “ages” of majority if you think about it.
Minimum age to be President for example.
There’s many others. Milestones set by age of sorts.
But regarding the OP, yeah, it seems the age of majority is creeping back up to 21.
Set it in a different historical period, probably future, and on a different planet, with either humans (which gives some restrictions) or alien (which gives no restrictions).
Another option is to an alt.timeline. Start with a premise: “If” some historical event swayed in the favor of youth rights/liberation rather than against, how different would the world be today?
You’d have to rewrite a LOT of history for that though but it’d be an awesome read.