Many of us (me included) all agree that riders should use their common sense and work their way up to the more powerful stuff but have a look at this....
If you took your test on a 125 in December 2010, you are restircted to 33bhp until December 2012 when you can go out and ride anything. Till then you can only ride up to 33hp (a Fireblade with a restrictor kit is allowed).
If you take your test in Feb 2013 on a 125, you can only ride a 125. You then have to take another test for 35KW (about 40bhp) once you reach 19. You cannot automatically ride it and you cannot ride a restricted fireblade now, you can only restirct a bike with less than 70KW to start with. Once you reach 21 you can take yet another test to go unrestricted.
If you decide not to go for the 125 test (at 17 or 18) or 35KW test (at 19, 20, 21) , you cannot go for the 35KW test until you are 24!