Basically everyone does the same test and the category is dependant on your age the type and engine size / power output of the machine you take the test on as explained below.

At 16 years old you can only ride a Motorcycle / Scooter of upto 50cc with a maximum design speed not exceeding 50 km/h (approx 31 mph). This is classed as a Moped and is shown on the licence as category “P”. 

17 to 21 year olds have the option of mopeds or learner motorcycles / scooters. A Learner motorcycle has the following features: Engine capacity upto 125cc, with a power output not exceeding 11kW (14.6 bhp).  Anyone passing their test on a 125cc bike gets a category “A” endorsement that is restricted for the first two years to bikes of no more than 25kW (33bhp).   In plain English - you can ride a motorcycle of any engine capacity as long as it does not exceed 25kW (if in doubt ask your local dealer) and if it does exceed 25 kW then the power output will need restricting to the prescribed limit.  And just to confuse matters further if you do not require to ride motorcycles of above 125cc with a power output of no more than 11kW (a standard 125cc bike) then you can take a test on a bike between 75 and 125cc and end up with the sub category “A1” licence.  So in other words you have a full bike licence for motorcycle of bikes upto 125cc / 11kw. If you ride any motorcycle above this capacity it must be done as learner motorcyclist with a DSA instructor.

21 years and over allows you any of the 3 options above and what is known as Direct Access (DAS).  DAS gives you the option of training on a higher powered motorcycle (at least 35kW) and on passing your test you will be able to ride any capacity of motorcycle you require.  You can only ride the higher powered motorcycle whilst in the presence of and undergoing training with a DSA approved Instructor.  This gives you a category “A” endorsement on you licence without the 2 year restriction.

One final point for those people who have taken the standard category “A” bike test and ended up with a 2 year restriction, if you reach 21 or are 21 and over, before your 2 years are up you can take a test on the higher capacity motorcycle and upgrade to the full derestricted category.

The technical jargon explained:

bhp – Brake horsepower (symbol: bhp) is the measure of an engine's horsepower (power output) without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components such as alternator, power steering, and AC compressor. The term was originally used to calculate the amount of the work steam engines could do against horses whilst ploughing a furrow. For example if it took a horse 10 minutes to plough a furrow and the stationary (or braked) steam engine could do it in five then the engine was said to be as strong as 2 horses

kW – Kilowatt - The kilowatt (symbol: kW), equal to one thousand watts, is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is roughly equivalent to 1.34 bhp.