Warren Sanderson & Sergei Scherbov had a very interesting article in Nature earlier this year (you can find the full article reproduced here on page 5). The article title really tells the story in itself: average remaining lifetimes can increase as human populations age. Put differently, we may be facing the interesting enigma that the longer we live, the longer we have left to live.
But, riddles aside, what Sanderson and Scherbov actually propose is a new metric: the median age of the population standardized for expected remaining years of life. Now why would that be interesting?