To fund an expense that can easily balloon above half a million dollars, tax-sheltered medical savings make sense, and they are within reach for affluent consumers in their middle years. In 2015, HSAs will have annual contribution limits of $3,350 for singles and $6,650 for couples, plus a $1,000 additional catch-up contribution allowed each year for those over age 55. (As before, contributions are made in pre-tax dollars.)

At current contribution limits and an annual return of 4 percent, a couple could begin investing at age 45 and save nearly half their likely out-of-pocket retirement medical expenses by the time they turn 65. Likely increases to annual contribution limits, as well as the $1,000 annual catch-up contribution after age 55, could help them save even more.

The account's earnings are tax free, as long as they are used to pay for items on a long and generous list of medical expenses that include everything from hearing aids to health-related travel. Unlike funds in a flexible spending account, money in an HSA rolls over from one year to the next and so has time to compound and offer returns on investment.

In the past, Stahl says, “most people have thought of HSAs as a way to get a tax deduction and fund items that insurance doesn't cover.” By putting more money in than they spend in a year, or by tapping other funds for out-of-pocket costs, consumers can invest HSA money now and have more—sometimes much more—to spend later.

“If you can fund your medical expenses out of pocket while you are still working, you can reach retirement with a pot of gold socked away in an HSA,” Stahl says.

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