One of the things I talk to people about in the fire service, both for fire prevention and when we go on service calls for smoke or C.O. detectors, is how they only last about 7 years. I show the homeowner the manufacture date stamped on the back, and if it is older than 7 years or is so old it doesn't have a date I recommend immediate replacement.
We built our house in 2004/2005 and it has 6 hardwired Kidde FIREX smoke detectors with 9V battery backup. I had two crap out needing replacement years ago, but the other four were original. I finally followed my own advice and replaced them all with brand new Kidde FIREX i4816AC models since every one was older than 7 years. They run on 120VAC and use 12-3 wire daisy chained to every receptacle box. The red lead in the 12-3 is a signal wire that if one goes off, they all go off. The red LED tells you which one alarmed if there is no visible fire. Of course, each one needed a new mounting plate and plug-in pigtail. Since I have an electrical background this was easy-peasy and all six were swapped out and tested in a little more than an hour. You should definitely call an electrician to do such a swap if you don't have the skills or ability to do this.
Those detectors are placed where they are per code and wake us up first. I also have an additional smoke detector on each floor tied into my home alarm system to alert the fire department in case we're not home. Those still have time to go before needing replacement.