I must be misunderstanding what Steam is… I nearly bought Age of Empires 2 through Steam, thinking it would be running *in* Steam, so my OS didn't matter. But, reading some comments, I found that some folks are looking for a port of the new HD version to Mac?
I must be missing something
Steam is basically just a store and multiplayer communication tool. The games still run as their own software, Steam just launches them.
I'm not a Mac user, but I guess there's some Windows emulating software for Macs (Bootcamp I think is one). It might be possible to run a game as old as Age of Empires through that