I think your fishing.
Level 40 should be enough due to the extra health, even some get more stamina at 40. Also there are class specific areas, why not run the highest level you can in that area?
Interesting take, I seem to always see everyone on lowbies when we do bosses instead of their high levels. Why is that?
Personally I do not have a 40. But yet some classes are better suited with party. And depends on the person, what they want to play.
Just because you get bored, doesn't mean someone else is/will.