I personally would say the UK has it harder, however the USA leaves drivers better prepared. The UK test is quite hard, because one mistake in unfortuante circumstances and your ****ed. Eg, If you forgot to look left for traffic and there was a car coming (even it it was miles away) youve failed. If theres no car there, then you get a minor.
The problem is, a fair % of UK people manage to squeeze through the test, and because that test was all that mattered, they decide that they can go out and drive at 80MPH through the centre of town, and kill someone. American's are a lot more educated on the matter and are less likely to act retardedly as a result.
Also the fact Americans start earlier is a good thing, the younger you are the faster you learn. Children can become sufficiently fluent in a language in 6 months, I doubt any of us could learn a language to that standard in 6 months. So being 15 is a decent advantage.