Is it possible that aging is a means for individuals to signal fitness?

Only if you're going to carry on reproducing at that age.

BTW, I may have misstated the method of breeding for earlier cessation of aging. I can't find again what I was reading that I described in those words, but other papers of Rose talk about promoting longevity by selecting for late offspring.

I found this bizarre too. So I looked up a paper by the same authors who wrote the book in the "aging stops" hyperlink to investigate.

By aging they mean the increase in mortality rate as a person gets older. Ie an 80 year old is more likely to die this year than a 60 year old.

The theory is that there is a given high rate of mortality that would prevail for the whole of life if natural selection did not exist. However, natural selection does exist and so for the early part of an organism's life the mortality rate is lower than it would be otherwise. This lowering reduces over the reproductive part of the organisms life. Thus before reproductive maturity the lowering is maximum, and at the end of its reproductive life the lowering is gone and the organism reaches the (high) unadjusted level of mortality. Thus species that stop reproducing earlier reach this high level of mortality earlier.

From our perspective, and using this definition, it would be better if "aging stopped" later, because then we would live longer before reaching the higher mortality rate.

Of course, what we really want is to simly reduce that higher mortality rate permanently.