20 is probably good for most things. The problem comes from trying to apply one age to all people. I wouldn't want rough inner-city types to be able to purchase guns at age...14 let's say. But out in the country where I lived I'd trust even a 14 year old with a firearm if they were raised around them and understood the maturity required to handle one safely. Same goes for driving, some kids go out hotdogging their ride well into their 20's. I drove like a granny until I was 22 or so until I was perfectly comfortable driving in any situation. This question is too tough lol