I replaced a smoke alarm earlier this year. It was in the house when we moved in 8 years ago and was very sensitive. It would go off at the meerest hint of my toast starting to burn. As it was getting on and as I wanted to put some extra units in I replaced it with a new one. I also have noticed that I get fewer alarms with the new ones although they do work but seem to need a little more smoke/fumes before they operate.
I also replaced my CO alarm last year as the old one started indicating that it was at the end of its life. CO alarms only have a 5 year life and will start beeping intermittantly when this time is up. I found one that uses AA batteries rather than PP9 and these seem to last just as long and as I usually have some available rather than have to go buy a PP9 its far more convenient.