All alarms wear out over time. Replace smoke alarms by the date shown on the unit. Manufacturers typically recommend every10 years. However, if an alarm is not working correctly, or is damaged, replace it immediately.
When choosing an alarm, consider where the alarm will be installed.
Ionization type smoke alarms typically respond first to fast flaming fires. They are best suited for rooms that contain highly combustible materials (e.g.,  cooking fat or grease, flammable liquids, newspapers, paint, and cleaning solutions)
Photoelectric type smoke alarms typically respond first to slow smoldering fires and are less prone to nuisance alarms near the kitchen area. These models are best suited for living rooms, bedrooms and near kitchens.
For maximum protection, install at least one ionization and one photoelectric type smoke alarm on each level of your home
Other tips:
Test smoke alarms monthly.
Change the batteries in your alarm twice per year. Don’t use rechargeable batteries. Unlike regular batteries, they lose their charge without emitting any warning signal.
Gently vacuum once a year with soft brush.