So .. the thing is, this is a descriptive account of the biology of the brain. However, I sometimes see the "discourse machine" building narratives around pushing the age of majority later, and I suspect this will get used in ammunition for normative purposes.

Just when the "brain doesn't finish developing until 25" nonsense has finally waned from the zeitgeist, here comes a new pile of rubbish for people to latch onto. Not that the research itself is rubbish, but how they name/describe the phases certainly is. The "adolescent" and "adult" phases don't have any correspondence to what we normally think of as those developmental periods. That certainly wont stop anyone from using this as justification for whatever normative claim they want to make though. It's just irresponsible.

Seems a stretch to use this as the basis of any radical change like raising the voting age to 32 (although maybe it supports reducing the minimum presidential age from 35 to 32!), but it does perhaps suggest looking at what kind of soft-paternalistic structures might help “adolescents” make better life choices. It is a little absurd that we expect an 18 year old to navigate the world with the same competence as a 40 year old.

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.

The older people get, the less they want youngsters to vote, drink, and drive--unless they believe they have something personally to gain from it