I don't understand why it isn't, just look at whats changed for motorbikes, if your under 21 (was talks of changing it to 24 I think) your restricted to 33 bhp for the 1st 2 years before having to take another test... why not do that with cars limit people to say a 80bhp car for the 1st 2 years then have to resit your driving test see how many fail 2nd time round.
It has changed now its along the lines of
16 is 50cc
17>19 125cc max 14 bhp I think
19<24 is 46.6BHP or two years experience which ever comes first after 19
And over 24 do as you like
BUT I'm 21 done it in august (changed January) and my licence has no restrictions on it , just says full cat A
So I don't think even the DSA knows what's going on anymore