Thanks for your thoughtful response. You partly convinced me.

it might be easy ... to show a correlation

I believe even this is very hard to show since it is more an effect over decades and it is an indirect relation. On top of that there are many other factors in play.

Causation would indeed be even harder to prove. However it still makes sense to me logically.

he prime working age participation rate is increasing... I am not sure how this is bad for the economy.

No it would be good of course. That is why the fact that prime working age is still far from pre-crisis levels is a problem and if you follow my logic you would be even more worried because you would actually expect it to be higher than the pre-crises level if there would be a full recovery.

One issue you run into here is that there are far more prime working age people than there are retirement age people. Moreover, there is a reason we care more about the statistics for prime working age people. Old and young people tend to drop out of the labor force for reasons that have nothing to do with the health of the economy. Mainly, they retire or go to school. The prime working age participation rate gives you way more information about the business cycle. If we are trying to figure out to what degree the economy is in expansion or contraction, narrowing the demographics like this controls for an important confounding variable.

I completely agree

Unemployment has been trending down and prime working age participation has been steady or increasing.

The American economy is clearly doing better than before. However, even though prime working age participation has been increasing it is still far from pre-crisis levels and places some doubt on the extend of the recovery.

I mainly am focused on the long term trends worldwide and I am worried that the developed world is entering a period of stagnation.