I know at least with Rome, they take a while to have kids. Barbarians, no. Realistically it probably was the other way around, Julius Caesar said that for the Germanians, to have had knowledge of a woman before the age of twenty was a disgrace, and the longer you waited, the more prestige you had.
you have to use what generals are around
Quite so. Later on, if I don't get good generals, I will just take an additional heavy cav and use him as the captain. Roman generals usually have to work for it, though I have seen a general come of age with seven stars, don't ask what traits he had as I don't remember. Military Genius was probably one of them. Those kind are very rare, usually I don't see higher than three or four.