A1 licence category – At age 17 before passing your test you can ride a small motorbike of 125cc (up to 11 kW power) whilst displaying L plates after you have completed CBT. You cannot carry a pillion or use the motorway. If you pass an A1 test you can ride your 125cc machine without any of these restrictions.
A2 licence category – At age 19 you take an A2 test which will allow you to ride any motorcycle of a power not exceeding 35kW, with a maximum power to weight ratio of 0.2 kW/kg, not derived from a vehicle of more than double its power. This all sounds confusing - but the reality is that there are loads of motorcycles which fit this category or can be restricted to fit this category. They will typically be middle sized machines around 500-700cc – your local Motorcycle Dealership will be able to help you choose a motorcycle that meets this specification and provide any advice you might need.
A licence category – At age 24 you can take the full A test and then you can ride any motorcycle. (This licence category can also be gained at the age of 21 if you have held a full A2 licence for 2 years)
If you are upgrading from one licence category to the next licence category you will not have to complete another CBT or theory test again as long as you wait 2 years in between test - this is known as Progressive Access.
It is much easier than it sounds – speak to a trainer who will help you. For more information on riding and age restrictions you can follow this link. Have a look at this easy to understand table (Routes-to-Licence-Poster-V10-low-Res.)