For baby boomers who manage their own nest eggs, a risk is looming that has nothing to do with stock prices or interest rates.

The risk is cognitive decline, which can rob them of their judgment, often without much warning. One big mistake—or a series of smaller ones—can go unnoticed by loved ones, and potentially ravage a lifetime of hard-earned savings.

To mitigate these risks, there are things baby boomers and others can do now to prepare for any problems. In addition, big do-it-yourself investing and trading venues like Vanguard Group, Fidelity Investments and

Charles Schwab Corp.


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are strengthening some of the ways they detect possible signs of decline. Among other things, all three firms check for clients’ difficulty navigating security protocols or need for frequent password resets. In such cases, a designated family member might be informed.

Vanguard also checks client-call recordings for keywords—such as “confused”…

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