In most states, the age requirement for a driver's license is 18. There's nothing wrong with that, but some states probably want to raise it because of high accident rates among teenagers.
Not really, people need to go to work and stuff when they're that age [sooner, actually] If they want to cut down on accidents they could make the tests harder, and search your record of if you get drunk often or do other reckless activities.
"or if you seem immature"
That's the problem. They can't just tell you that, and there's no way to confirm a claim. So they rely on tests, and of course points on your driving record.
I think it seems reasonable on paper (or in text on the internet, lol) but in reality, that hinders a lot of people's lives. For example, in most states, you can get a job at the age of 16, but now they can't drive. It's a hassle.
Don't hide behind the internet and act like someone you're not. It's uncool.