I recently gave some family and friends a test. They all failed. I asked, "Have your smoke detectors expired?" The answers were mixed but pretty much amounted to, "I'm not sure."
The bottom line is: you should completely replace your smoke detectors after 8 - 10 years of use. So guess what I did after Christmas? Yep. Good thing, too, because 1 of my old detectors actually had malfunctioned.
So the house is now equipped with brand new dual sensor alarms throughout. And if you're wondering just how tardy I was accomplishing this important task, let's just say it should have been done 6 years ago. --face palm--