The sensor of a CO detector is quite sensitive, and that sensitivity decreases with age. CO detectors therefore have a limited shelf life, which varies from detector type to detector (and from sensor to sensor). The lifetime of a CO sensor ranges from 5 to 10 years. Always check the product information to find out how long the sensor lasts.
The CO detector will give a signal when the end of the sensor's life span is near. In that case, replace the CO detector completely immediately!