system
A chat on subreddit /r/PoliticalCompassMemes
human
Simple solution: once you are an adult (18 years of age), you are a sovereign individual and should be afforded all the rights and responsibilities that adulthood comes with. 

If you think 18 is too young for drinking, firearm ownership, or gender transitioning, then maybe they shouldn't be considered adults at that age and allowed to serve in the fucking military and whatnot. 

Devils Advocate: The problem with going by age is that people go through puberty and mature at vastly different rates, and that some are simply more intelligent and mentally mature than others regardless of age. So maybe instead, dependence can be the deciding factor; they can only be considered adults when they are no longer dependent on a parent or guardian- which would also open adulthood to emancipated teenagers (for better or worse). Or, maybe the hard limit of 18 should be kept as a way to force people to become independent and mature as adults.

EDIT: I'm not arguing for or against non-adults transitioning here.
gpt
I say keep that solid line of 18. Cuts out all the grey areas and jumping through hoops.