AFPS 75:
If you serve for 16 yrs from age 21 as an officer or 22yrs from age 18 as an OR you get your pension immediately. If you leave before these points, your pension is preserved. Preserved pensions are paid out at age 60 in respect of pensions built up prior to 6 April 2006 and 65 for pensions built up from that date onwards. You can ask for any pension due at 65 to be paid as early as 60 but it will be reduced by 5% for every year it is paid early and the lump sum will be reduced by 3% for each year it is paid early.
AFPS 05:
If you serve until 55 you pension is paid straight away. If you leave before that age it is preserved for you until you are 65. You can ask to have it paid as early as 55 but it will be reduced as described above.