I'd assume that at the rate of one year per 12 turns it'd take a real turtleplayer for national troops' aging to be a problem.
More serious it would be with mages and such you'd might see stay alive for long... ...and honestly, I'm itching to see some useful but really short-lived troops. They could even implement it so that Devils' contracts on earth last only a specified time, like a couple of years, if those guys are seen too powerful. Okay, and that was total speculation.