Carvalho, Ferrero, and Nechio have a new paper (pdf) on this question. Here is one interesting sentence:
An increase in longevity or expectations thereof puts downward pressure on the real interest
rate, as agents build up their savings in anticipation of a longer retirement period.
Here is another:
We calibrate a tractable life-cycle model to capture salient features of the demographic transition in developed economies, and find that its overall effect is a reduction of the equilibrium interest rate by at least one and a half percentage between 1990 and 2014.