In games and movies we have many different varriations of the same thing. People trying to kill eachother en masse. There are many different versions of this, for many different games.
We have the Past Battles: Battles that occured at some point in history, or had similar troop types. This starts from the dawn of Man's "intelligence", the Stone Age. From this point, it ranges up to modern technology, and beyond. At this point, it becomes the Space Age theatre of warfare.
Space age warfare seems to be more abundant then modern or past warefare. I'd estimate it'd only be another 20-30 years before Age Of Empires has to start reaching into futuristic battle, assumeing the series lasts that long.
Futuristic Warfare reaches into places that no Human can traverse... For now. We still suffer from one very large problem - the lack of FTL Drives.
As such, they reach into places that people can only dream about, as our knowledge on planet formation is mostly limited to our own solar system.