Heres a chart showing how it is now and what they are bringing in,
http://www.90-one.com/testchange.html
Basically, If u want to ride say a full powered 600cc sportbike before 21 (I heard in another artical it was going to be 24, But we will find out when its actually in place) then you will have to pass the bike test before jan 19th, Because of your age, You will have to be restricted to 33bhp for two years (Currently what im on, 33bhp is still plenty of power for a first timer)
Passing the test before jan 19th will be very hard (theory, mod 1, mod2) especially on the run up too Christmas and alot of pressure!