The sensor in a smoke detector is a tiny tiny tiny amount of the radioactive element Americium 241. Since the half life of Americium 241 is over 400 years, I doubt that they "wear out" in the traditional sense of the word. My guess - and it is only that - is that the sensors eventually get coated with airborne stuff that forms a film over the sensor - stuff like grease, oil, water-vapor-caused rust, or the like. This alters the airflow, and the sensitivity of the detector, to the point where it is no longer reliable. Although the interior of the smoke detector could be cleaned, it is easier and surer to lay out $5 for a new one.
I could tell a sad story about a rust-caused smoke-detector alarm - but unless you were involved, you'd be bored.
Bottom line, MisterP is right. Check the date, and replace when expired. Mighty cheap insurance.
Bill
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