With the patch it says that it you can play it with vista:
The Microsoft Age of Empires 3 update 1.10 will fix the following bugs in the game:
* Support for Windows Vista with fixes for various capability issues
* Added a new random map, The Unknown
* Added ability to see if a friend on ESO is playing a game
* After each game players will now be granted bonus experience, meaning all Home Cities will now level up faster
* Players on your pest list will not be able to join your game if you are the host
* The experience requirement to level up Home Cities that are level 100 or higher has decreased
* New avatars available on ESO
* Moderators now have blue chat in chat channels
* Added Power Rating to ESO
* Added Stand ground to the game
* Attack move button and hotkey added
* Hispaniola, a new random map has been added
* Recorded game playback improved significantly
* Added Option for minimized multiplayer chat
* Added games list filtering options
* Added Option to save games list filter settings
* Refresh/Resume buttons added
* Exploit fixes
* Numerous balance changes
* Fixed an issue where a game would be cancelled and all players given a 'Failed to join' message when two routers that could not communicate joined the same game.
* New hotkey editor added to the game
* New large map set.
* Five new large versions of popular maps have been added.
* Fixed a sound crash issue.
* Fixed a bug with sound being choppy
* Fixed some performance issues with boat battles.
* Fixed a D3D initialization error when launching the game.
* Sound effects (and sometimes music) stopped playing
* Tweaks to movement code to improve the responsiveness of units
* Changed formations so that unit types keep their setting even when grouped with different types
* Postgame screen that allowed players' names to be edited
* All players would hear the selection sound for HC cards played
* ESO connection problem dialog box wouldn't close
* Command manager caused slow performance when CPAIs were in the game