between 35-45. i always let my generals lead an army right off the bat at 16 y/o. i assign them to a particular region on the outskirts of the empire bordering a hostile state(s) such as in taras, near the romaoi. of course they are carefully chosen, usually having sharp/charismatic/vigorous and loyal. i roleplay dull and/or langourous generals as dissenters/rebellious bastards and send them on impossible missions to roleplay executions or the defeat of a rebellion. anyway, by 35 my career generals have already fought nonstop (i tend to NOT steamroll enemies, instead allowing them to recover during a truce/forced alliance so that i won't ever run out of interesting battles to fight. the romani are a DELIGHT to exterminate) for 19 years, which is a pretty long military career that's provided each of them a long list of (heroic) victories. by their 30th year i start to look for a successor, by 35-40, i've usually found one, however there were several cases where my generals were forced to soldier on for an average of 10 more years because of a lack of 'worthy' family members (and thus a heap of 16-20 year-old already-dead men).

also, at certain times when a region is being attacked in many directions and it would be impossible to defend with only 1 army and general, a retired general could be recalled and an army given to him (i always have five small armies ('marines') stationed at sea, one each in the aegean, black, eastern mediterranean, ionian, and ligurian seas as emergency reinforcements.