I took my second group of 5 out to Northrend at 68ish after I got all the outland flight points and such. I am happy I did so. The items you get questing out a zone in Northrend are really a big upgrade even from Outland questing gear. You also get like 40% more experience per quest than you'd be getting in Outland. 20k is the standard exp for a normal quest there. I think I was only getting 11k at most in Outland.