“Smoke alarms that are 10 years old are near the end of their service life and should be replaced. Some people think that their smoke alarm sits idle until smoke is present. But it is working every minute, constantly monitoring the air 24 hours a day. For example, an ionization smoke alarm goes through 3.5 million monitoring cycles in 10 years. In a photoelectric smoke alarm, a light operates 24 hours a day to check for smoke particles in the air.

Just like any electrical appliance, the components of smoke alarms wear out over time. When a smoke alarm reaches 10 years of use, the potential of failing to detect a fire increases substantially. Replacing them after 10 years reduces the likelihood of failure.

Both the hard-wired and battery-operated alarms are equally affected by age.”

from National Association of State Fire Marshals http://firemarshals.org/rfsi/smokealarmfacts.html

7/14/14 the link above not longer works. Here is some updated information

“The life expectancy of smoke alarms is generally 10 years, after which point their sensors can begin to lose sensitivity. The test button only confirms that the battery, electronics, and alert system are working; it doesn’t mean that the smoke sensor is working.” http://www.consumerreports.org/cro/news/2014/04/when-to-replace-an-older-smoke-alarm/index.htm

CO & smoke alarm buying guide. Last updated: May 2014

http://consumerreports.org/cro/co-and-smoke-alarms/buying-guide.htm