I'm at 21 Legacy. I might ascend when I hit 25. Then again I might hold out for longer. I haven't experienced what you guys have yet with the wrinklers and all that so we'll see.
I have ascended three times already.
But here is that upgrade that triggers the Grandmacalypse. Its called One Mind. Once you research that, the background of the big cookie goes to a darker red tone, and the Grandma turns into a Goa'uld.
You need at least 100 grandmas to trigger the upgrade. The wrinklers start coming for the cookie and if you minimize the window then when you come back, as many as 10 of them will be munching on the cookie. But like Lord Ba'al said, if you pop them then you get zillions of cookies from them which represent the 5% of CPS they cost you.
Once you go for the upgrade, you get this warning:
If you say yes, that starts the Grandmacalypse. After a while, you can get the Elder Pact which stops it. And you can revoke or renew the pact when you want which is fun. There is a particularly satisfying "pop" sound when you kill the wrinklers.