These professors believe that

demographics are the most important factor in determining long-term market trends...[and] that individual investment behavior largely depends on age-related patterns. Younger adults, those from 20 to 39, are generally consumers. Middle-aged people, 40 to 59, tend to invest in stocks. Retirees are likely to sell more stocks than they buy.

They may be right, but I'm skeptical. Trends can change all too easily. Then again, if it's true, I guess we should privatize social security.