tl;dr let’s not worry about it. There are two effects: old people work more when times are bad (among some others) because their assets become worthless and some number of people can’t afford to go to school. This boosts labor force participation during recessions. With the aging of the population, this former effect conceals the drop in labor force participation due to aging. As the economy improves, we’ll see the aging/education decline + increases due to a better economy/better prospects – which is the difference between those two points on the graph at the bottom.