Prior to 2010 on AFPS75 you were awarded an immediate pension after 12 years service if made redundant. In the lead up to the SDSR redundancies the rules were quietly changed via a Statutory Instrument to 18 years service from age 18.
I can confirm that - I've been getting a pension since my redundancy date of 3 January 1977, when I was 30 years and 3 days old. As I joined when I was 17 that got me the 12 years service required for the pension from my 18th birthday. As an officer, of course, only the 9 years from my 21st birthday counted towards the rate of pension I received. That was a voluntary scheme and within a certain period you could pick your own date, so there was no coincidence in the date I chose.
It seems that things are a lot less generous now, and my son, a REME NCO counted himself lucky that he made it to his 22 year pension point last year before leaving, as some he knew were made redundant with the finishing line in sight.