Beth Bergmans liked working as a project manager for an online university based in Minnesota. “We are offering opportunities for people to advance in life — that brought some satisfaction,” she said. “And the people I work with are awesome.”

Ms. Bergmans, 63, planned to stay on the job for two years, until she qualified for Medicare. But in recent months, something had shifted, subtly. In her fast-paced workplace, she began to find it harder to recall the details of recent meetings, to retrieve words and to filter out distractions. She took short breaks at her desk to recharge.

“You find ways to adapt,” she said. “You use Post-it notes and whiteboards, and you spend more time prepping before a meeting.” Nobody complained or even seemed to notice, but Ms. Bergmans worried.

“People don’t really talk about this, the fear that you’re starting to slip professionally,” she said. “The last thing I want is to crash and burn at the end because I didn’t recognize that it was time to stop.”

So a couple of weeks ago, Ms. Bergmans told her manager that she would retire at the end of the year.

Sometimes it works this way: People engaged in and proud of their careers, intending to continue past typical retirement ages, encounter internal or external difficulties and step aside, even if no one is urging them to resign or retire.