At whatever level you can kill the hardest (upto about 7 levels above you, above that the difficulty vs. experience gained ratio tends really fast toward much more difficult). I find 65 at the farm camp in DSH to be one of the fastest places to gain AAs, but there are many others, depending on gear, current AAs, tactics, etc. Farm is just easy, especially with a tank merc, even with crappy gear, and a low dps toon.
AA XP is the same for a white con at any level. There is just a scaled bonus for low total AA count. And some mobs, as well as, zones, and other factors give experience modifiers. Level on its own has nothing to do with experience rate for AA now. Only reason level would possibly even matter, is due to availability of certain gear, spells, disciplines, and/or AAs.
Might want to look at this thread for more details:
https://everquest.allakhazam.com/forum.html?forum=1&mid=126428051345149414 Yther Ore.