I have read the other theories, and I find the reasoning lacking. If the Oath Rod was instituted by the Black Ajah, then since its members have bypassed the Three Oaths, do the get to live a lot more than the rest of the Aes Sedai? The same question goes for the theory that the Oath Rod merely shortens life.
Furthermore, we have not read yet an account of an Aes Sedai dying from natural causes, except from the Amyrlin foretelling the Dragon's rebirth - death caused by extreme stress? So, there is room for speculation.
What if the agelessness is purely a self-construct image, i.e. the Aes Sedai subconsciously choose to freeze their age? This may be due to the way they are taught to handle the One Power, or an effort to stand above and beyond the rest of the world - Mr. Jordan has provided us with many examples of such an attitude. They use the One Power to freeze their age, or slow it down considerably. The Knitting Circle definitely do not wish to look like an Aes Sedai, therefore they lack the looks. This effort may have as a result a shortening of the life span - you cannot look great AND live forever!
Now, as to the actual length of their lives. We can assume that, from all the groups of One Power wielders, Aes Sedai lead the most dangerous lives, therefore they do not get the chance to grow old.
I find it somehow suspicious that we were not given a clue as to how Aes Sedai die from natural causes. Perhaps Mr. Jordan has an ace up his sleeve.
wotmania says: I do not know for certain whether or not the Black Ajah instituted the Oath Rod (but I do think that would be a cool twist). But I would be willing to bet that the Oath Rod does shorten the lives of Aes Sedai, and cause that ageless look - remember when Graendal mentions that the Aes Sedai of this Age bind themselves? You might have a point that Aes Sedai lead the most dangerous lives out of any of the Power users in Randland, but I would say the Wise Ones are right up there, too...

True Power is knowing that you can, but you don't. 
-Deathbone1 
What About...