Ancient. This may be due to me being a compulsive restarter for a long time, but I think it has more to do with expansion being fun. You expore, and found your cities without competition. All you have to manage is getting roads to everything and racing to get settlers out with juuuuuust enough protection not to get invaded. Of course, I imagine expansion would be more boring for people playing on random maps. I basically play huge 60% pangea exclusively.
I'm suprised people like to war in the Middle Ages. If I could, I'd use that time just to build since I can't expand anymore, but when I play on emperor stopping to smell the roses isn't really an option.
In the industrial ages you get everything built, and have nothing to put shields towards but units. That's when you go to war. Of course, at that point it sucks since you have to use stacks of 20 units to get metros, but at least it's not the logistical nightmare of huge pangea war in the middle ages when all you have are roads.