This blog has mentioned couple of times – ignore demographics at your own peril (see my paper on this).
This new paper from FRBSF econs - Zheng Liu and Mark M. Spiegel – says demographics could lead to lower stock market indices in future. As baby boomers become baby busters, they would be moving away from equity as an asset class leading to its decline.
They develop a ratio – middle-age cohort, age 40–49, to the old-age cohort, age 60–69- and call it the M/O ratio. This M/O ratio depicts P/E ratio of S& P index really closely since 1954: