For the longest time, I ran lvl 18 challenges with a lvl 20 character solo (i will fail somtimes at lvl 20 so 18 is the guarenteed ingreds run).
I've been lvl'ing up a new alt, and been running lvl 10 -12 challenges at lvl solo. Whoa, 1) they are much easier 2) they give much more ingrediants, more than double.
It's faster to farm ingreds at lvl 12, bank them, pass them to my lvl 20 to craft epic gear. Plus, all the while the lowbie is getting xp. Very tempting to keep the alt as a farming toon for a long while.