Depends on what faction I play...

As "Civilized faction, except Romani"
Usually I got only one or two generals campaigning, usually had 8,9 or even 10 command stars in his 45s, while they in the field at 60's, they usually "carry" much younger FM's (usually 17 or 18th, after some time sit in academia) with smaller armies behind him to act as his "student" (very seldom his son, usually grandson!!!), and take on his position (and almost all his military ancillaries) if he hit 70's... after that, the "retired" general usually sit in a province nearby his student to watch his action, and reinforce if necessary... (usually they got warmonger traits... so sitting them down in city isn't a good option)

As Romani...
Teleport the FM back to italy when he hit 65's, especially when he got triumph, after that, sit in some backwater town... or if he got "fatal" warmonger trait... stand until death in Italy...

As Nomads / Barbarians
Campaign until got a heroic death, with his "student" then briefly teleport to capital (act as delivering his body), and then continue the campaign...



And my generals who get "Scarred, Vigorous, Spartan, Seen the elephant" trait, usually life into his 90s with many girls arround him, while his less healthy brothers usually die peacefully in his 70's