Armies dropping off due to old age would be nothing sort of ridiculous. By what I know of it the Roman Legions were something of century-spanning institution unto themselves and stayed in the rosters and action for quite respectable periods. Just as a little reminder, the legionaires actually had a retirement age, a pension system and to boot were granted a small plot of land to settle (plus full Roman citizenship should they lack it) when their time was up. Just as new guys were brought in (sometimes through plain press gangs...) to replace campaign casualties they were also recruited to fill the gaps left by retiring vets.

As for the others, well, most of the nomads and barbarians were "citizen-warriors" anyway; for those peoples fighting was something of a way of life and every man above certain minimum age and status was supposed to join in when his cheftain demanded. And actual warrior aristocracies were flat out self-regenerating - war was both the priviledge and duty of such classes, and sons followed their fathers into the profession. Heck, they might well flat out inherit all the necessary bells and whistles to boot...

"Dying of age" for armies ? Poppycock.

Campaign attrition would certainly be a neat detail, but it'd also make the whole thing hideously complicated and frankly I don't think either the overall system or the operating logic of the TW series would be up to the snuff. I'd suggest turning to the Europea Imperialis series and its latest incarnation, Victoria, if you crave that level of strategic detail instead.