The image below shows the price earnings ration (P/E) and the ratio of the middle-age cohort (age 40-49) to the ok-age cohort (age 60-69). This is labelled as M/O in the chart. There is a clear correlation between the two series.
And if we look forward, this is what their paper predicts:
Their model generates a return of the P/E ratio to the low levels of the late 70's early 80's.
There remains, of course, uncertainty about this prediction given that it relies on many factors (such as demand coming from other countries, changes in retirement age), but the correlation of the first figure provides a strong enough argument so that the issue cannot be ignored.