Like a lot of creaks, I really thought this one was a bottom bracket creak. It started after about 2 years and about 3,000 miles of nice quiet performance on the bottom bracket. Creaks most loudly under stress - uphill mostly, but sometimes going hard on the flats.

It's a carbon fiber Felt Z frame with a threaded BB. Sram Red crank with a standard (non ceramic) Sram GXP bottom bracket.

What I've done:

Replaced the bottom bracket. The old bearing were indeed real gritty so I thought that would have solved the problem. I assembled it just as I'd done originally: cleaned everything well, greased all threads, used a torque wrench.

Creak was still there. It was there whether I was standing or sitting so I went into it further, more or less in the following order, none of which eliminated the creaking:

Removed, cleaned and re-greased pedals. Cleaned and lubed pedal mechanism (Time RXS). Re-tightened cleat bolts on shoes .... also tried my MTB pedals and MTB shoes (which I've been using on another road bike which doesn't creak at all).
Removed, cleaned and applied assembly paste to seat post (Ritchey carbon fiber post) re-lubed and re-torqued seat post clamp.
Removed saddle. Cleaned and greased rail-rail-clamp attachment. Greased and re-torqued rail clamps.
Removed stem and cleaned and applied new assembly paste to steer tube and handlebar and greased and re-torqued stem bolts.
Removed, cleaned and re-torqued all chainwheel bolts
Removed, cleaned and re-torqued cassette
Cleaned and re-lubed chain
Fine tuned rear shifting
Removed the BB again. Re-cleaned and this time I used teflon tape on the bearing cup threads instead of grease. Reassembled and re-torqued.

Any other ideas? I'm kind of at a loss.

Mine was a creaky crank set.

If you're my age, maybe it's your knees

It sounds like you've done a very comprehensive attempt at eliminating sources of a creak. I'd check the frame over inch by inch to make sure there are no cracks. Cracked crank? Any recent changes to components?

Does the creak happen all the time or just when you are pedaling?
Does it matter how much force you applying to the pedals? Is it worse on steep climbs or during sprints?
Does it creak when you hit a bump or rough patch of road?
Can you recreate the creak by bouncing your weight on the bike while standing still?
" " " " " " by pulling forward and back on the handlebars while standing still with the brakes locked?

Missed an important one. Tighten your wheel skewers. Sometimes they need to be super tight. Every time I hear what sounds like a BB creak on a ride, I'll pull over and tighten the rear skewer. 75% of the time that's it.