Oddly, I received credit for the Ancients quest when I was in act 5, but was in the Frozen Cavern when the rest of the party completed the quest. I was level 28 or so. I had just entered the game and had not gone anywhere near to any access area close to the Ancients. This has pretty profound game consequences if the Ancients no longer have to be killed, or accessed, by the player in order to advance to Baal. It actually defeats the purpose of the Ancients! Good luck, should be easy to repeat.