Driving is not difficult. Driving safely can be difficult for some people, but most everyone has the skills required for it. Unsafe driving is far more often caused by other factors than lack of skills for it. So the argument is not just about how skilled drivers are, but how safely they drive. This also means that testing the skills would not be enough, but some kind of psychological profiling bull**** would need to be done and those are not ultimately reliable (or affordable) means of testing for driving lisence.
Whether or not 16-year-olds have tendency to generally drive more dangerously than 18-year-olds, I have no research data, but I would guess that by average psychological profiling, a 16-year-old would be more likely to make fast and bad decisions and be more easily affected by peer pressure to drive fast and cool and ultimately unsafely.
I'm not saying that 18-year-olds would not be susceptible to same faults, but I am saying that they have two less years to die in a self-induced traffic accident before hopefully growing up and learning to drive safely.
Of course, the counter argument is that when driving becomes a normal thing that you don't need to wait till your 18th birthday, driving dangerously would "lose it's appeal", but I don't really buy it - it just moves the wait to 16th birthday, and then the 16-year-olds will be satiating their need for speed instead of 18-year-olds. And whether or not the "need for speed" reduces faster for 16-year-old new drivers or 18-year-old drivers, I would guess 18-year-old-drivers - and there would probably be less of 18-year-olds who do have this "need for speed", for lack of better wording.
Obviously some drivers never grow up. Conversely some never develope any need for speed... I was 19 when I drove my lisence, I took my time in clocking driving hours with our car when it was necessary to get from place A to place B (my mum got a teaching lisence so paying for driving lessons wasn't an issue). It was never a big deal anyway, and I don't even especially like driving, it's mostly boring, and when it isn't boring it's very very stressful.
Interestingly, most drivers rate themselves "better than average"... just an interesting statistical anomaly.
