(FORTUNE Magazine) – How do companies live to a ripe old age? It helps to avoid nostalgia, be flexible, or sell insurance. Of the ten oldest U.S. companies identified by Dun & Bradstreet's business information service -- excluding farms, cemeteries, and church enterprises -- three sell insurance and always have. Explains Daniel Crough, president of 209-year-old Mutual Assurance in Philadelphia: ''General stores tend to die out when the proprietor dies. But a good insurance company has to have very long-term commitments.'' Another predictor of longevity: unsentimentality. The companies that have lasted know how to shed musty, unprofitable businesses and find new ways to turn a buck. Dexter, in Windsor Locks, Connecticut, began in 1767 as a grist mill. But as demand for milled grist fell off, the company began producing specialty papers for stationery and later for tea bags. Today Dexter makes, among other things, adhesives and coatings for aircraft -- $1 billion worth last year. Says spokesperson Ellen Cook: ''We have no traditions whatsoever. None.''

The country's oldest company, J.E. Rhoads & Sons, started tanning leather for buggy whips in 1702 -- and everyone knows which way that business went. Fortunately, when the Industrial Revolution got rolling in the 1860s, Rhoads began making conveyor belts, and it lived to tell about it. One company that hasn't changed: D. Landreth Seed. The Baltimore firm sold vegetable seeds to Thomas Jefferson (whose estate still buys from Landreth) and George Washington -- to whom it extended 30 days' credit. What kind of seeds did the founding fathers favor? Landreth's president, Benjamin Goldberg, 90, doesn't go back that far. ''I'm pretty old too, but I don't remember.''

CHART: NOT AVAILABLE CREDIT: FORTUNE TABLE/SOURCE: DUN & BRADSTREET CAPTION: AMERICA'S OLDEST COMPANIES