"It is not enough to survive and have children. Your line has to continue through your grandchildren.
In this context, the survival of women beyond menopause makes sense if it translates into extra grandchildren. Proving that hypothesis, though, is difficult...
...The average lifespan of post-menopausal Finns was 68. Of Canadians, it was 74.
These ages correspond to the points where the matriarch's children themselves had stopped reproducing." [via Economist]