I think the party line is that it starts 1 Apr 04, you join the PAS when you reach your 22 year point, and you must remain on the PAS for a minimum of 5 years.
It should be open to all FS and MACR, and initial offers are made at around 18/19 year point.
The rates are the same as Officer PAS, but the max levels you can reach are rank dependant.