The A category of motorcycles and scooters can be ridden at 24 years of age once you have passed the full A test. This will involve the motorcycle theory test, a CBT certificate and a test with the D.V.S.A. The minimum size bike you must take the test on is 595 cc and must weigh at least 180 kg.
The current A category was introduced on January the 19th 2013 and can only be taken by people of over 24 years of age.