I'm not sure what the code is that causes manufacturer's to restrict household ionization-type smoke detectors to a life of 10 years. However, similar detectors made for commercial installations do not come with a stated 10 year life-span printed on them.
From a scientific perspective, the radioactive isotope used in ionization-type smoke detectors is Americium-241. This has a half-life of 432.2 years. That means that after 10 years, only 1.6% of the americium-241 would have decayed to neptunium.
And I seriously doubt that the ionization detector can distinguish between 100% activity and 98.4% activity. But I guess it is possible the 10 year life-span might reflect aging of electronic components rather than the isotope itself.
In my mind, the bigger danger with smoke detectors is that people don't change batteries, so I view the requirement that they be hard-wired as a good thing.
