What all that adds up to is that by 2030 there will be barely three Americans of working age for every person over 65, compared with a ratio of five to one today. That's going to be a significant challenge to Social Security. It might also be a problem for stocks.

To finance their retirement, folks over 65 will have to sell assets, but there will be relatively fewer young Americans to buy them.

This shift could certainly be another long-term drag on your returns. Economist James Poterba of the Massachusetts Institute of Technology guesses that it might cost investors an average of a quarter to half a percentage point per year.

But don't waste your time listening to any market guru who promises that demographic trend lines can tell you when to hop out of stocks to miss the boomer-driven crash. There may never be one, Poterba says.

The so-called age wave is a widely known and easily tracked statistical event, and the market has a way of calculating information into prices long before any zero hour arrives.

Several forces could blunt the impact of boomer aging. For example, Jeremy Siegel, author of "Stocks for the Long Run," has argued that a rising middle class in India and China will seek the safety of U.S. equities. And boomers surely aren't going to try to sell all at once.

Some will hardly sell at all. A big chunk of this country's assets are in the hands of the richest investors. "Most wealthy people save a high percentage of income," says Roger Ibbotson, founder of Ibbotson Associates and professor at Yale School of Management. "They aren't really digging into their savings when they retire."