Interesting, so it seems like the best strategy when you get an army in the ancient and medieval age is to save them until the modern age. Once you load a unit onto an army then it cannot be unloaded right? So if I load up elephants onto my army then they are stuck there and cannot be replaced by armor later on?
Once you have a Military Academy, you can build armies without requiring leaders. To build a Military Academy, you must first have one army that is victorious in combat.
Therefore, when I get that first leader, I would almost always immediately create an army (because you can't get more military leaders from battle while you have one unused one), immediately load up that army with my most damaging units, and immediately try to win that first battle so I have the prerequisite for building the Military Academy out of the way.
Once you have the Military Academy available, I don't think there are situations in which it would make sense to save armies for later. Use them now, be successful now, exceed your current military goals. When those armies become later obsolete, let them get destroyed and replace them with new armies from your Military Academy.
Maybe if you were at the tail end of a war and very close to a new technology allowing significantly better units, saving the army for later would make sense. It isn't cheap to build armies with your Military Academy, you certainly don't want to waste armies. But I think it's a very limited set of circumstances under which you'd save an army for later rather than using it now.