The Garrisons are available to level 90s when they upgrade to the Warlords of Draenor expansion.  Blizzard has said they will be available “within an hour” of launch, which likely means there is an opening questline to do in order to start building your Garrison.

Please log in to rate this.
people found this helpful.

At this time, each character will have its own garrison, similar to how farms are unique to each character.

Please log in to rate this.
people found this helpful.

While alts can’t share a garrison with your main garrison, Blizzard has said there will be a way to make it easier to level an alt’s garrison.

Please log in to rate this.
people found this helpful.