From San Fran Fed

This evidence suggests that U.S. equity values are closely related to the age distribution of the population. Since demographic trends are largely predictable, we can forecast the path that the P/E ratio is likely to follow in the next few decades based on the predicted M/O ratio. Figure 2 compares the actual and model-implied P/E ratios for the sample period ending in 2010. We calculate the path for the model-implied P/E during the sample period by feeding in actual M/O ratios.

Projected P/E ratio from demographic trends