The main problem for insurers when deciding the risk for 17 year olds is that there is very little consistency throughout the age bracket. If you take a group of 60 year old men, they probably drive much the same. This is not the case with 17 year olds.
When I was 17 I would occasionally do things like get some major oversteer on corners and speed as fast as my 1 litre would take me. It impressed my mates no end, because none of them could get oversteer. Despite that, I would usually be pretty safe and was always very alert, looked far ahead, braked early and only drove like that on empty roads at 3am.
However, I knew people who tried similar driving, lacked the coordination and wrote their cars off. I also knew people who wouldn't drive at more than 30mph and were pretty frightened of driving. And then there was a friend of mine who managed to rip the front wheels off his mum's car because he drove whilst stoned and forgot to steer for a corner.
Now, an insurer has to come along and stick all of those drivers into one bracket. Sure, the majority of 17 year olds don't have or cause an accident, but the insurance company is going to cover their own bottom by charging excessive premiums for all. This has the added effect of reducing the number of 17 year olds who get their own car.
This drove me mad when I was 17. I didn't get my own car because the maths were: 1 litre car, £1000. Insurance, £2000.
Kind of off the point, but I was wondering. This whole Diamond "aren't girlies so great behind the wheel" policy. Imagine if there was no such thing as an insurance excess, and CCTV camera were in every supermarket car park. You could claim for the (albeit cosmetic) damage every time someone clips your car with a trolley, opens their door into yours, or even that old favourite, rests their trolley against your car because the car park is on a slope. Now, you would have to extend the car insurance to cover you when pushing a trolley of course. But I reckon that women's premiums would go up above those of men's.