You must be at least level 82.
There is a portal in your class hall that leads to Deepholm
- Returning to Deepholm
After completing the initial quest line, you can teleport back to Deepholm from the portal hubs in Orgrimmar or Stormwind.
actually you can get to deepholm at level 81.
Stonecore instance unlocks at 81 and can be selected for specific que. my tank was in a group and we wiped. i didn't know where the entrance was, so i did a GY res. i was in deepholm able to start doing quest there after my res-sickness wore off. i did this on 17 Oct 2013
my players XP was turned off so i could update gear to then turn XP back on before doing leveling BGs. i was still able to do 106 of 110 quests in the area at level 81, unlock therazine and ended up revered. there was a level-82 drop quest i could not do, and more than likely the other 3 quests that would complete the area must be from the normal way of doing this area at level 82
i used portal to stormwind and the portal there was working to go back to deepholm at level 81