I would love to know this too!

If I'm doing a level 15 story quest on my level 27 Pugilist, I'd imagine that I could do it in the bits and pieces of not-great gear I have on hand (a level 20 chest piece, dated level 24 shoes, etc) as I could likely take on level 15 enemies easily even with my character wearing the starter gear. But -- does that level 20 chest piece scale down to a level 15 chest piece of relatively equal strength, or does it scale down based on how good it is for a level 27 character?