There are several versions of Age of Empires available on Steam, so whether or not it is free depends on which version you are referring to.
Age of Empires Online was available on Steam at one point, but it has since been removed from the platform and is no longer playable.
Age of Empires II: HD Edition is available on Steam for purchase at $19.99. This version of the game is a remastered version of Age of Empires II.
Age of Empires III: Complete Collection is also available on Steam for purchase at $39.99. This version of the game includes the base game as well as the two expansions.
Age of Empires: Definitive Edition, Age of Empires II: Definitive Edition, and Age of Empires III: Definitive Edition are all available for purchase on the Microsoft Store, but not on Steam.
So, in summary, there is no free version of Age of Empires currently available on Steam.