[thejeff]

I like the upgrade with age idea. Only applies to some units, obviously, but there are enough obvious upgrade paths in the descriptions that it's worth thinking about.

Even mages: Daughter -> Mother -> Crone?
Obviously balance would have to be considered. Might make more sense if only old and experienced units upgraded?

How about a nation that couldn't recruit its top mages, but had to wait for the lower tier ones to age? And hope they didn't die or get crippled first.