What factor limits the lifespan? There are no moving parts. Half-life of the radioactive sensor or something?
Combination of sensor life and build up of dust in the sensor path - as I understand it, anyway.
I've had smoke detectors last 20+ yrs and still test fine but after the publicity on life span, I replaced them. The white plastic case goes ivory, then yellow, so you know it's far too old.
Tarzan, that's what I found in my research. Slightly higher up where the heated air is but not right near the ceiling, not in drafts, not near where kids can touch it, etc.