Do you all think the fact that the human brain isn't fully developed until age 25, should affect the way we think about adulthood and maturity?
Start exerciaing at an early age, and you will be well developed.
To contrast with 'modern' expectations, we were expected to help on the farm from the age when we were capable of doing something useful. As we got better, we got new duties, taught to do more complex tasks involving greater responsibility. We viewed this as a rite of passage, and amongst our peers, we gained status by being allowed to do these more complex and demanding tasks.
Our privileges were based on our demonstrated ability to be responsible and use good judgement, something which comes with practice, and is not magically conferred at a given chronological age.
At 9 I was deer huntiing, by ten I had been driving tractors on the farm for five years, could take the skiff out and go fishing. At 14 I held a commercial fishing license, and was a member of the local VFD. I went on to become a geologist, making decisions or advising those who did affecting tens of millions of dollars in projects a year. My peer group included an Navy Seal and later fire chief, and a Sergeant Major in the USMC--all of us who were taught at an early age to be responsible. Long before my 18th birthday, I was taking the boat out to the river blind and laying in decoy spreads so my friends and I could hunt ducks.
My point is that being responsible and exercising good judgement, and when situations call for it, restraint, comes with practice, and that is something that isn't being answered by waiting until the brain is fully developed to start doing, any more than athletes are developed by waiting until they are a certain age of full development to develop the skills they need to compete.
When you consider the average life expectancy of a person in the era from 1750 to 1800 was 36 years,
http://www.legacy.com/life-and-death/the-liberty-era.html, someone who was 21 then was like someone who is in their 40s today. But long before that they had often used firearms for hunting and defense (from humans and animals), often were already married and parents, and commonly working their own farm by 14-16. Those "Free, white, 21, male, property owners" were the equivalent of today's thirty to forty something business owners, not the 21 year olds of today who have been, frankly, held back in their civilian development by the establishment of arbitrary age limits, yet are considered old enough to hump 100 lbs of gear and a rifle in the desert somewhere.
