I don't think it is the age of new drivers but their newness.
I knew someone who when she passed her test her dad bought her a car, something like a Volvo, a big car. His reasoning was that she would probably have an accident and a bigger car gives you more protection.
I think there are things that can be done to lessen the risks to new drivers.
I have a motorbike licence, there are restrictions on the type of bike you can ride depending on your age.
For a moped you have to be 16.
For a 125cc you have to be 17
For a 245 it is 19 and to get an unrestricted licence you have to be 21 and go through the other catagories or 24.
Maybe we should look at something similar for car licences?
As for not having motorway experience, there is nothing to stop a new driver taking a lesson on a motorway after their test.