Quote Originally Posted by antipope
I installed 1.1 midway through my campaign. Perhaps 1.1 wont let me build units that in history may not have been invented yet?
That's probably it. The unpatched version of M:TW contained a bug that allowed the player to construct units of all era's.

Quote Originally Posted by Brother Mark
Don't certain units also become obsolete as the game progresses?
True. Some units will disappear after a certain date. For example you cannot recruit Varangians any more in the Late era.