I love the idea, and it would add alot of realism.
I thought 10% sounded a bit high at first, but after running some calculations and comparing to real military re-enlistment rates it's quite generous due to only effecting undeployed crew basically on the waiting list, while real militaries like the US has similar amounts retiring every year. (approx 64% average re-enlist roughly every 4 year = 10. 6% total retirement per year for everyone including those deployed ).
I think this is great idea, although we should be careful not to model it too closely to current (as in Earth) organization. Think of of those poor auxiliaries of the Roman army that had to serve for a minimum of 25 years!
You can also picture a caste-based society where you will be a soldier for life, starting as an apprentice gunners assistant, working his way up to master gunner, and, once age start setting in, getting relegated to guarding the ships cleaning cupboard.
Or a feudal-type society where the CO of the ship bring his own crew from his estate and holdings. The crew will be a mix of military professionals (pilots, marines, gunners) and former civilians (engineers, cooks, medical professionals)