I chose the Aztecs to play with, because I wanted to see the fearsome jaguar warriors in action. They have a low attack, but since they can withdraw, they´re not so useless, especially considering how cheap they are. But by the time I was building up my military, they were obsolete! I think they should remain available at least for the duration of the ancient age. This way they are like chariots: obsolete before you use them.