I took my priest/4 mages into RFC at level 14 and ran it until level 18 or 19 without too much trouble. No full wipes (healer died a couple of times doing pulls) and the occasional mage death while AoE'ing, but nothing major.

I tried Wailing Caverns at 19 or 20 and it was HORRIBLE. I could single target dps, but if I AoE'ed and the jackasses would call for help and then wipe-ola.