Currently, the lowest level at which you can pass through the Dark Portal and go to Outlands is 58 (although it is possible to be summoned or portaled to Outlands at any level). However, is it better to go straight for the portal when you hit 58, or are the quests going to be too hard, and would it then behoove one to wait until 60? Personally, I'll be taking all my alts in when/if they get high enough, because the quest rewards are just overwhelmingly good. Aeus agrees with me:
- Outland would be your ticket there... did it with one of my alts; best thing I ever did..