Here is the full spec. I think from Jan 2013 in the UK
Age 17 or 18
From 19th January 2013. You can take your test now on a 125cc and that's what you are limited to until the age of 19, when you can train and take another test if you choose - otherwise restricted permanently to 125cc. There is no automatic progression!
Age 19 to 23
From 19th January 2013. You may take your test now on a 400cc or restricted bike (min 395cc - between 33 Bhp and 46.6 Bhp). When you pass then you can ride a restricted motorcycle producing no more than 46.6 Bhp Eg. older Kawasaki ER500 or a middleweight bike that has been professionally restricted. NOT however large bikes that originally produce more than 92 Bhp - this means pretty much everything over 600cc is excluded from you that is a modern sports bike or sports tourer. Also the power to weight ratio is set at a maximum of 0.2 Kw/Kg - this means light or smaller cc sports bikes are not allowed too.
Now wait 2 years or until you are 24 when you can train and take another test if you choose. There is no automatic progression!
Age 24 or older
From 19th January 2013 you may take training and pass your test on a 600cc plus motorcycle that produces at least 53.6 Bhp (much more powerful than the minimum today). Pass your test on this and you have a licence for any motorcycle, any size, any power.