Quote Originally Posted by Jabberwock View Post
Heroic dungeons are only in the queue if you are at their expansions level cap.
This is true if you're looking to do a random, but you can still specifically queue for heroics of the prior expansion up until the dungeon's max level range. So, usually expansion max level +2 (e.g. Cataclysm heroics can be queued for manually from 85-87, except for Stonecore for some odd reason).