I know that in the US "adulthood" via voting rights was extended to those from 18 up from the age of 21 in 1971 with the 26th amendment to the constitution in response to the student activism against the Vietnam war... a good portion of the argument had to do with the fact that if you're old enough to be drafted, fight and die for your country you should be old enough to vote (the age at which you could be drafted was 18)
In the rest of the world the voting age was lowerd from 21 to 18 diring the 70s for a good part, though I'm not as sure as to the cause why because I haven't studied other countries constitutions as much... hope that helps some!
*** it might not, because mine gives too much info on American History
Human beings make life so interesting. Do you know that in a universe so full of wonders, they have managed to invent boredom? -Terry Pratchett
There has to be some age where we can assume that most developing children have developed their own cognitive ability enough to make competant descisions and life choices. That age is probably reviewed frequently by the professionals in that field too, and when they provide a claim that we should change the age limit, politics takes over. Politics is the 'weaker link' though, no guarantees that they'll even do anything with the info, because it (obviously) takes effort to change these things.
Well, anyway, that age limit may not be perfect, but we don't live in a world where perfection occurs at the drop of a hat. To be on the safe side, that age could be moved up to 21 or something, but that would probably cut population growth by some significant amount I presume .
It is completely wrong and inaccurate to look at the age of maturity as beginning on your 18th birthday or your 21st birthday or any birthday. Like the beginning of puberty is usually between 11 and 14 for boys and 10 and 12 for girls, it just doesn't begin at one single age but is different depending on gender, race and other factors. The same holds true for brain development too.
It is very inaccurate to think that on your 22nd or 25th birthday your brain just finishes maturing and that is that. Gender, race and individual differences play roles on when the brain matures fully. Most scientists would say the brain usually finishes maturing from any where from 23 to 26. There is also evidence that adolescence ends around age 20 from sleep studies showing changes in sleeping patterns from teenagers to adults. It is known teenagers need more sleep than adults and this is a good socialpsychological indicator for ealry adulthood. Saying the brain matures around 24 or 25 is just an average and is not true for everybody.
If I had to choose one age for maturity though it would be age 20. It is not as low as 18 but not high as 21, so an age between 18 and 21.
It is also the beginning of the twenties and sleeping patterns usaully change in the early 20s.
This is still not satisfying because it is not completely accurate but like king-of-despair said there has to be some age where people can assume to be competent to make good decisions. Another thing about human development is females usually mature faster than males in both puberty and brain development by about 1 to 2 years average. I wonder is there an evolutionary reason for this?