Thanks for sharing your thoughts.

I agree that proportion of elderly people is rising in the US population. However, this only partially explains the decline in participation rate. In fact, even if you take out 65+ population from total, the participation rate still shows a similar trend.

Moreover people are working longer. Therefore one should include 54-64 cohort also in calculation of participation rate and not leave them out. I find the two metrics that you mentioned useful but not comprehensive enough. The first one ignores people aged 54+ and 25- while the second one ignores those out of labor force for more than 12 months.