Most smoke alarms, battery-powered or mains powered, have a 10-year life, after which you should change them to make sure you have the best protection for your home.
If someone else installed your alarms but didn't leave you with the installation date check for the replacement date on the alarm itself.
For British Standard EN 14604:2005 certified alarms the manufacturer must put their recommended replacement date on the alarm's labelling in addition to a batch code or date of manufacture.
This information may be located on the side or back of the smoke alarm.