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Coldharbour access is available directly through the main storyline at about Level 43, and quests and enemies in the zone start at Level 44, so if you are a lower level you will need to be well prepared, but note you will also not get appropriate rewards from those quests and enemies if you are below ~Lvl 40, as rewards are capped at 5 levels over. Out of interest, what is your reason for wanting to get to Coldharbour early?