So if an ancient one starts out with when he stirs in his slumber, this happens….when exactly will an ancient one stir in their slumber?  Just by being the ancient one in play? OR does something activate it?

"Stirs in his slumber"

The phrase implies the Ancient one is sleeping or in the process of awakening.  When the final battle starts (at the end of the game), the Ancient One has awakened.  So the effects that take place while the Ancient One stirs in his slumber are in effect during the entire game.  Think of them as influences from this particular Ancient One on your game.

Hope this helps!

It is simply a particular choice of words, nothing more. The Ancient One continuously slumbers (stirs in his slumber) until he awakens. So the Ancient One stirs in his slumber during the entire game right up to the point when the final battle begins.

The slumber ability affects the whole game, including Final Battle (there is no way you can regain the max San / Sta lost due to Cthulhu's slumber ability, for instance). From the FAQ, pag 13:

Q: Does the Ancient One’s “Stirs in his Sleep” ability stop once the Final Battle begins?
A: The ability is in effect during the entire game, including the Final Battle.

Thanks……….  I added periods since it said my post was to short…….

It has always bothered me a bit that the AO apparently still stirs in its slumber even when it is awakened.
But on the other hand, why would some of its powers disapear when it wakes up? So I guess its actually just the wording on the AO sheets that bother me, not the mecanic itself.
 

Well, normally, if you just pat the AO on the back, that gets it to settle back down again. Works on infants that are getting fussy so why not giant abominations from beyond the stars?

Julia said:

Cthulhu awakens! Gain 1 sanity and 1 stamina. =P

Haha, no, I haven't played the game like this. I had effects such as Abhoth's, Nyogtha's and others' in mind when I wrote my answer. Those don't interfere with the final battle. But Cthulhu's effects do indeed. Thanks for pointing that out.

Two slumber abilities that probably should not work in final combat are:

Quachil Uttaus: his dust deck cards probably aren't meant to be drawn anymore.

Revised Hastur: If an investigator is devoured, is he also considered to be in Arkham, thus increasing the terror level (once it goes past 10 he'll get doom tokens back!)? I've eventually settled that in final combat, investigators are not in Arkham or an Other World or a location or a street, but they are in the same space for the purposes of Paranoia.