You're uneasy because you're a frequent advocate for policy positions that most people tend to be weary of once they've accrued some amount of life experiences (although the experiences and speed of acquisition differ from individual to individual) hence anything that casts shade upon the decision making of the youngest adults is a potential threat to your goals, if only a theoretical and circuitous one at that. If such policy positions do not survive life experience are they worth advocating for? 100yr ago people has been treated by the world around them as adults for at least half a decade by the time they got to vote at 21. Why not do the same today?

Regardless, specific policy implications are totally beside the point.

The problem with this "well you're not akshually an adult until X" stuff is that it is basically a re-hash of long out of fashion "women are hysterical, blacks have big muscles and small brains" type crap from 200yr ago that was used as a justification to continue preventing these people from finding their own way in life unbounded, consequences and all. First off, the logic is flawed and self referential, of course housewives and slaves couldn't adult, they never had the opportunity to gain the experience, same with 22yo college kids you're measuring today. Removing the racism and sexism by simply applying it to everyone doesn't change the flawed logic. But that's not even the big problem. The big problem is that at a societal level you're reducing the number of person-years available for full adult capacity work and productivity. You can solve this with coercion (state, social norms, etc), but people are less productive when they're not working for themselves so you're still handicapping your own society. A society that does not encourage people to develop and become adult and achieve and produce at full capacity as quickly as possible (which is likely a different timetable in an agrarian society than an industrial one, details left as exercise for the reader) WILL eventually be outcompeted by one that does, though a head start may buy time.