My co-worker recently bought a new home and when I asked him what he was up to this weekend, he said he was heading over to Home Depot to buy new smoke detectors. When I asked him why, he informed me that smoke detectors "expire". I had NO IDEA! My home will be 10 years old in 2014 so I'm now planning to replace mine as well.
For an easy to read (and follow) guide on smoke detector maintenance check out this site.
Another thing I learned, once a year you should take a vacuum to your smoke detector to clear particles out of it so sensors function effectively.
This right here is proof that you learn something new every day - hope it helps you!