“If you can detect emerging financial impairment early, you can also step in early and protect the person,” said Daniel Marson, a neuropsychologist and director of the Alzheimer’s Disease Center at the University of Alabama at Birmingham. “It may be if you step in two months from now, they won’t be in a position to make a poor decision or be exploited a year from now.”

For Ms. Clark’s father-in-law, Francis Taylor, the intervention came too late. At 80 years old, he married a woman 17 years his junior, who, over their three-year union, according to the family, cashed $40,000 in blank checks sent by his credit-card issuer and emptied the contents of his $123,000 annuity, leaving him with little more than a giant tax bill.

Mr. Taylor, a former diesel mechanic and Korean War veteran, gave his wife permission to make two annuity withdrawals over the phone. But his wife, who couldn’t be reached for comment, made 20 more withdrawals on her own by using her husband’s Social Security number and other identifying information, and signing papers to direct money into a joint account, according to documents provided by Ms. Clark. After an internal investigation, MetLife, the annuity provider, concluded that it had followed proper procedures.

Preventing these situations is often difficult. Knowing exactly when to get involved can be fraught, whether you are an adult child or a trusted adviser. There are a series of early warning signs of financial decline, which Dr. Marson identified in a recent study, which is being submitted for publication and was funded by the National Endowment for Financial Education and the National Institute on Aging.

The signs, while perhaps not surprising, are subtle, making them easy to miss: It may become more difficult for people to identify the risks in a particular investment, and they may focus too much on the benefits. Completing various tasks on a financial to-do list may start to take longer, such as preparing bills for the mail. Everyday math may become more laborious or prone to errors, whether that’s figuring out a tip in a restaurant or doing a calculation that requires two steps. Financial concepts, like medical deductibles and minimum balances required in savings accounts, may also become harder to grasp. Naturally, these behaviors should represent a significant change: If a person was never adept with personal finances, this won’t serve as much of an indicator.