However, as birth rates fall to below replacement level, a bulge enters retirement and starts drawing down on their savings, or to greater detriment, they draw on social security schemes that must now be funded by a smaller work force. These macroeconomic trends weigh down the economy. Even before these older workers enter retirement, they are highly paid, but less likely to be innovative risk-takers in ways that grow the economy.
The whole piece is worth a read, its an excellent primer on the subject. So what's to be done? McArdle calls for the usual remedies of raising the retirement age and forcing people to save more on their own. These are in fact good ideas, but I wonder why no one ever touches on the idea of immigration from countries where there are still high birth rates or there are two many people to countries with demographic issues. The U.S. has a demographic issue of its own, not as bad as that of Europe. We are in a better position to absorb immigrants than Europe, because we are already a nation of immigrants. I have posted this graphic before, but look again at the advantage of bringing in workers from a country like India:

