radiodude
Dancing Dog is on the money
Look at the lifestyle involved with being a military aviator. Talk to some current soldiers if you can find them. More importantly, talk to some EX-soldiers and ask them why they left.
Same-same for the Navy.
But in answer to your question:
IMHO, 31 isn't too old. But much older than that and you're going to have to have something extra for you to be considered competitive. I did my assessment up in Tamworth in 2005 (at age 42) and was accepted. But I was most likely accepted because I was a current serving member (Chokkos) was very fit and had an extensive and varied army background. (I declined, but that's another story).
DIVOSH!