One needs an A licence to ride a motorcycle. Since the introduction of the third licence directive (19 January 2013)  the motorcycle licence is subdivided into three categories : A1 (for light motorcycles), A2 (for medium motorcycles) and A (for all motorcycles).
Motorcyclists under the age of 20 years old obtain the A1 licence after having passed the theory test and two practical tests[i] successfully. The A1 licence, allows them to ride a light motorcycle with a cylinder capacity up to 125 cc (maximum power of 11 kilowatts [kW]). For more information see the (archived) SWOV Fact sheet Category A1 (to 125 cc) of the new motorcycle driving licence.
From the age of 20 years-old motorcyclists who have the A1 licence can obtain the licence for a medium duty motorcycle (A2 licence; maximum power of 35 kW), by passing an additional practical exam. After 2 years in the possession of the A2-motorcycle license an additional practical exam can be taken to obtain the license for a motorcycle with unlimited power (A).
Taking the exam for A2 or for A directly (via the theory test and the two practical tests[i]) is also possible, provided that the minimum age for that category is reached. For A2 this is 20 years old and for A this is 24 years old. More info on the motorcycle licence and the legal requirements can be found at the site of the CBR or the national Government (both in Dutch).
[i] The practical tests consist of one initial test vehicle control (AVB; this certificate has a validity of one year) followed by a traffic participation test (AVD).