I've been looking at this unit from Lowes:
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www.lowes.com/pd_429793- ··· =4113870I bought my house in November 2012. It's 1985 construction and I bought it from a woman who was in her 90's. So, I have no idea how old the smoke detectors are, as everything else I asked her about ended up being wrong (because I think she just couldn't remember).
So, I am going to replace the three units in my house because I have no clue as to their age. I pulled one down and couldn't see anything that would give me an indication.