They probably should have made all accounts under 50 private.
18 is pretty much only reasonable when it directly aligns with a legal requirement. Otherwise it is not the most reasonable, but the most lazy choice.
https://www.urmc.rochester.edu/encyclopedia/content.aspx?Con...
>After infancy, the rate of myelination slowed during childhood and adolescence, but exhibited continued growth until the end of the third decade. Specifically, mean MFLD during later adulthood (defined as ≥28 y) was significantly greater than in adolescence and early adulthood (11–23 y; Welch t, P = 0.000059; Mann–Whitney U, P = 0.00016).
https://www.pnas.org/content/109/41/16480
Our (dorsolateral) prefrontal cortex is one of the last parts to myelinate.
>The last areas to myelinate are the anterior cingulate cortex (F#43), the inferior temporal cortex (F#44) and the dorsolateral prefrontal cortex (F#45).
https://en.wikipedia.org/wiki/Myelinogenesis
This is the area of the brain that handles things like planning and abstract reasoning (though it isn't the only area involved in such thinking).
Yes, to the extent we accept it so much we made it into law. Though the law does seem to pick a few different age groups for other things based on a seemingly arbitrary standard. Lower for driving, even though bad driving could kill innocent bystanders. Higher for smoking, even though smoking only kills yourself. You can even sign up and join the military a year before you are able to vote for the politicians who control it.
Anyway, I'm pretty sure people are allowed to stay children for longer nowadays, instead of being told to marry and work in the coal mines in their early-to-mid teens.
More cynically / conspiratorial / on the other hand, people are kept infantilized as well as a means of control by various parties in whose interest it is to keep a population docile and dumb. Can't overthrow the government if you've got scantily clad teens to watch.
For example in Britain (age of majority 18, 16 in Scotland)
Sex: 16
Marriage without parental consent: 18, except Scotland (16 for local elections, 16 for Wales coming in next year)
School leaving: 16, but must remain in education outside of school (say official apprentice) until 18
Drinking: 18, 16 in public with a meal when bought by parent, either 0 or 5 at home
Driving: 16 moped + tractor, 17 cars, some vehicles are even higher (23 or 24 for powerful motorbikes for example, 21 for minibusses unless in the army)
Smoking: 18 to buy, 16 to smoke in E+W, 18 to smoke in Scotland
Gambling: 18 (lottery was 16 until recently)
Voting: 18, except in some caes in Scotland where it's 16