My info on O2 sensors comes from Bosch and Bumper to Bumper auto parts propaganda. Also from an article I read in Hot Rod or Car Craft a couple of years ago.
Apparently the sensor gets clogged, but will work to some degree (or variety of degrees) when it gets some age on it. Until it goes kaput that is.
But, it's made to be a throwaway part like a sparkplug.
According to Bosch and Bumper to Bumper, to keep a car running to it's utmost emissions and mixture capabilities, the sensors should be changed every other oil change.
According to HRM/CC mags, if you have a computer controlled car that is bein hot-rodded or run hard, then changing the sensors often makes sense.
In higher end cars like Ferarri (Modena) and Porsche (911,935, etc) they have it on the regular dealer maintenance schedule.
FoMoCo does it on their flex fuel cars.
But, since the sensor is the eyes and nose of the computer for enrichment/leaning purposes, it makes sense.
I change the sensors in my Girlfriend's V6 LeBaron about every other and it runs like a top.
A mopar top, that is.:-D