In the stock market, a "catalyst" is a development that an investor expects will influence a company's future growth. Right now, few catalysts loom larger over Wall Street than the aging of the baby boom generation.

The baby boomers were born between 1946 and 1964, and the oldest among them will turn 65 next year. As this huge cohort of Americans moves into old age, demand for various products and services will grow or wane.

Some opportunities will shrink …
For instance, many have predicted that as more people move out of the workplace, car sales will diminish, hurting Ford (NYSE: F) and others. Retirees rarely need two- or three-car households, and they put fewer miles on their cars when they're not commuting. A 2007 paper on travel patterns of older drivers cited figures showing that seniors tend to drive about half as many miles as younger drivers. But interestingly, car purchases by those 65 and older now represent nearly a quarter of all auto sales, up from just 12% in 2002.

Other companies that serve the non-retired may also suffer. Coffee sales are likely to feel pressure, since older people tend to cut back on their consumption. That could hurt business prospects for Starbucks (Nasdaq: SBUX) and its competitors. A 2007 study found that people aged 35-64 consumed the most coffee. Although the 65-74 age group still spends more on coffee than the average household, coffee consumption dropped by 44% by the time retirees hit age 75.

… While others will grow
Meanwhile, other companies have reason to welcome the aging of the Boomers. The biotech company Dendreon (Nasdaq: DNDN), for instance, has developed Provenge, a vaccine against prostate cancer. Of those who have the disease, more than 80% are age 65 or older. There's a bit of a question mark around the treatment, since it costs around $93,000. Insurers and Medicare are reportedly balking at the high price tag, but Provenge may still win out. Geron (Nasdaq: GERN) has a newer drug in development to treat prostate cancer, and it's also addressing diabetes, another condition that many older folks develop.

The health-care field stands to benefit in many ways. The average age for a knee or hip replacement is 67, and some 85% of pacemakers go to patients 65 and older. Older people will need joint replacements and pacemakers, which will boost business for companies such as Zimmer Holdings (NYSE: ZMH) and Medtronic (NYSE: MDT), respectively.

Keeping your eye on demographic changes can help you zero in on trends that may help or hurt your investments.

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Longtime Fool contributor Selena Maranjian owns shares of Medtronic and Starbucks. Ford Motor and Starbucks are Motley Fool Stock Advisor choices. The Fool owns shares of Medtronic. Try any of our investing newsletter services free for 30 days. The Motley Fool is Fools writing for Fools.