But that problem (Daughters->Mothers-Crones) is widespread. Many nations have similar progressions, though usually only 2 steps. Warlock Apprentice->Warlock,
Augur->Augur Elder, etc. Ermor can recruit Triari, aged veteran troops.
A mechanic for changing unit type with experience or age would be interesting, but it would be a drastic change, probably best saved for Dom4.
It's just part of the basic assumptions of the game. You can recruit units at various stages of training. Like many things, it doesn't make sense if you look at it too closely, but it works fine as a game. So fix the things that don't work.
I actually like the idea of a weak, but cost effective capital-only mage. If only the strong mage wasn't also capital only, so you'd actually bother to recruit Daughters.