Imagine a society with 100 people. A working adult can create 50 widgets per year, and there's 15 kids and 5 retirees. The economy produce 80 * 50 = 4000 widgets per year, which gives a per capita income of 40 widgets. You can have whatever tax, welfare, or income redistribution policies you like, but there's only 4000 widgets to go around.
Great. Now let's say 5 new kids are born, the 15 existing kids become adults, 30 adults retire, and the 5 existing retirees die. Bonus: we got 3% better at making widgets, so a working adult makes 51.5 widgets. We now have 100 people, 5 kids, 30 retirees, and 65 working adults. Total widget production = 65 * 51.5 = 3,347.5 widgets per year, or a per capita income of ~33.5 widgets. Again, policies can change the distribution, but not the total number.
What we're seeing is that if an aging workforce lowers the overall workforce participation rate, as a society, we get poorer. If productivity increases, as a society we get richer. It's just a question of which change is larger, and in the US (and Europe, and much of Asia) the answer is the aging workforce. The demographics are clear and brutal.
The most critical metric is the ratio of current workers to retirees; that number is climbing and is going to continue without a policy change that somehow reduces the number of retirees, or increases the numbers of workers. Large scale skilled immigration might do the latter, but failing that, we're basically out of ideas.
This article spells it out fairly nicely I think: https://www.bloomberg.com/view/articles/2016-10-10/innovatio...
