If you are 16 you can only ride a moped or a geared bike but they must have a maximum capacity of 50cc. A CBT gained on a moped is also valid for a motorcycle once you are 17.
If you are between 17 and 19 you can only ride 125cc motorcycles.
If you are between 19 and 24 you can ride motorcycles with a maximum power output of 35kW (46.6 bhp).
Once you are 24 you can ride any size of motorcycle.
For all of these you need to have passed the appropriate test first – see what tests do I need.