In co-op, when does the level difference XP penalty kick in? [duplicate]

Possible Duplicate:
How does experience work in groups of heroes with different levels?

I know that, if you're playing with a character who's significantly higher level than you, you receive a penalty to XP gained from killing monsters. But how big does the difference need to be to kick in?

I've seen 5 levels kicked around, but I haven't been able to find any credible source for the information.

Here's a question from a developer Q&A mentioning the penalty, but no specifics:

Guest-236: If i am higher level then my freinds and want to play with them, will there be any kind of handicap? for example will they get less exp
JasonBender: To Guest-236 There is an XP penalty for killing monsters too high/low in level relative to your character. It scales based on your level, so at lower levels you can fight a broader range of creatures. This is good because player levels tend to be farther apart earlier in the game.