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  According to Brooks, who was president of a think tank before he left to teach at Harvard in his mid-50s, one way to avoid sinking into midlife despair is to step gracefully onto the rising wave of your crystallizing intelligence and notice all the new doors of opportunity it opens.  “You don’t need to change jobs or careers,” he insists. “But you can think of it as moving from your cowboy curve onto your coach curve, where you’re now incredibly good at helping other people do amazing things.  If you’re a start-up entrepreneur when you’re 30, you can be a venture capitalist when you’re 60. If you’re a star litigator at 35, you can be a managing partner at 65,” he says. “In every profession, there’s a version of it—helping other people to become better at what they do—and it’s really rewarding.”

  But psychologists have identified two types of intelligence that flourish at different stages of life.  The first is fluid intelligence, which helps young adults innovate and solve novel problems and typically peaks in our late 30s.  The second is crystallized intelligence, which enables older adults to use the knowledge they’ve acquired in the past to identify patterns and educate others about the intricacies of complex ideas and systems.  And it’s important to understand the difference because people who assume that it’s curtains for their career once their fluid intelligence wanes may actually behave in ways that hurt their performance and health. Indeed, controlled clinical studies of so-called negative self-stereotyping have shown that when older adults are presented with negative age stereotypes and apply them to themselves, they perform more poorly on memory tests and even walk more slowly.