I got my license as soon as I possibly could. It was also through this process that I found out I needed glasses (at least, to a serious degree). At first, I could easily say that I wasn't overly responsible. If I had other people in the car with me, I would drive carefully, but until I got pulled over a couple of times, on my own I drove ludicrously fast (thankfully I have good reaction speed).
I started out with driving an SUV (a Highlander) - so I got hard mode. After eight years though, I could say I'm a whiz behind the wheel. This is a good thing as I live downtown and I have to commute out of there each day - and despite how conservative you drive, you
need to have some degree of aggression in order to successfully drive in a downtown area.
What Artie's saying is definitely true for some avenues of life, but not necessarily true for driving. There are definitely factors that are external to a person that can affect how you perform behind the wheel. When you're younger, you don't feel like you have a lot to lose and be responsible for like when you do when you're older - relatively speaking. I'll guarantee you that - in general - a woman who has an infant and is a first time driver will not try to be reckless on the road when compared to a young girl who just wants to meet up with her friends on time for some social event. There are still those oddballs now and then, though.
With regard to being able to go off to war, it's kind of a situation that solved itself for the betterment of the government. As I mentioned before, when you age, you feel like you have more to live for, so you're less likely to join the military. Moreover, when you're older you tend to become more educated - and more educated people want higher, better positions. There are very limited positions like that in the military ("too many people want to be heroes, and not soldiers"). Therefore, being able to enlist at that age and with that amount of education allows those people to have the means to make a living, and keeps the forces of the nation stocked.
In the end, regulations are the way they are because too many bad eggs spoil the omelet that is society. As such, you try to find that sweet spot that minimizes the detriment to society, while not being too restrictive at the same time. It's not perfect, but it's the best we've got so far.