IMO, that is the major reason for the decline we've seen...and the reason why it has flattened recently....also, as the FRED says, more research is needed in order to understand why it isn't increasing enough...hopefully you have some ideas on this...for me, it makes sense to INCREASE immigration, given our low birth rate which is unlikely to change markedly anytime soon...make sense to you?
Link: https://fredblog.stlouisfed.org/2017/06/the-recent-evolution-of-labor-force-participation/?utm_source=series_page&utm_medium=related_content&utm_term=related_resources&utm_campaign=fredblog