While Fur is right in stating the obvious, it can be a pain when there is a level discrepancy. If you go to a new zone some toons can't get all the quests, etc. What I do is pick up the collection quests you usually skip on those lower toons. I used to do a bit of out of group leveling (tag with the lower toon and burn with the others not in my group) but I got nervous when folks started getting banned for taking this too far. It may have been nerfed to hades anyways.