I'm sure this question has been asked before... so, sorry

My two lowbie moonkins are now 68 and ready for Northrend. I would like to use my 3 80 druids to level them through instances, however I'm not sure the level/instance that the experience penalty goes away. Anyone know? Am I thinking about the experience gimping correctly?

In other words, I'm assuming the experience for two level 77s would be the same in a level 78 instance even if I had 3 80s as compared to 3 77s. What is the lowest level instance where this dynamic holds true? I thought I remember reading if the mob is 9 levels below an 80 (ie level 71) then it is still green to the 80 and everyone in the group will get full exp. However, if the mob is more than 9 levels (70 and below) then the exp will be gimped.