You know this may actually be exactly what the RAF want. Join at age 18, leave (through choice) at 40, and have a young (fit) fighting force without any accusations of ageism. With the present (& future?) workloads, everybody will be burned out by 40 anyway, if not before. If you want a job with long term potential, join as a physiotherapist / trick cyclist.
You could then get ex-military (civilians) to do the instructional work and second line duties. Oh.... silly me; we already do.
The weak link would then be recruitment, not retention. I think the days of short term solutions (FRI) may well be over.