Hi Robo
If you are under 21, you can still hold a PCV Licence but you are restricted in driving over 50km. It would depend if the Arriva Co you were going to work for was urban or rural, some bus companies employ from 18.
As for the time limits, there is nothing in law that says you must hold an ordinary licence for a specific period of time before you can sit your PCV test, but you need to have a medical, another theory and the driving test.
As suggested Arriva will advise you of their policy.
Hope this helps
CDB