There is undoubtedly a dodgy period when you first pass your test, but the difference between a 17 year old male who's just passed and a 21 year old male who's just passed are noticeable. Sure DfT have done studies as part of their road safety stuff, I just can't be arsed to trawl back through their archives!
We're one of relatively few countries who just give you a full license and let you get on with it, most have curfews, limits on passengers or engine size, bans on certain roads etc, that's probably a better solution IMO.