ask the car insurance companies: above a certain age - way above 18 usually - their rates suddenly drop dramatically. The insurance companies don't make that age up: it comes from their accidents statistics.
It's pretty clear certain age groups get more into accidents than others: it's because they're not really mature enough to be good drivers, even after years of driving experience. Nothing reveals immaturity in a person more than their way of behaving on the road.
I'm saying this as a general rule of course: clearly there are good young drivers and incompetent old timers. But for the population in general, the insurance statistics don't lie.