Most people buy long-term care insurance in their 50s and early 60s. The cost goes up with age, but it's still affordable for many people over age 65. Once you hit the mid-70s, though, the cost of a good long-term care policy becomes very expensive, and it may be difficult to qualify for if you already have health problems.

Even if you're are in good health today, there's a good chance that you'll eventually need some type of long-term care, at least for awhile. By 2020, roughly 12 million people over the age of 65 will require some long-term care, according to a study by the U.S. Department of Health and Human Services.

But that only gives a general picture. The hard part is figuring out in advance whether you'll need a long period of close monitoring -- daily or even round-the-clock care -- that you'll have to pay for. Some 70 percent of the elderly don't pay for their care but get it exclusively from family and friends.

The odds of needing two years or more of extensive, paid care is not high. Long-term care insurance, then, is security against a small but nonetheless real possibility of a lengthy, expensive period of care.