In comparison, he noted, “men above 40 years of age” are useless. As for those over 60, there would be an “incalculable benefit” in “commercial, political and professional life, if, as a matter of course, men stopped work at this age.” Although such views did not prevent the doctor from going on to accept a post at Oxford University, one he retained until his death at age 70, his contention that brainpower, creativity and innovation have an early expiration date was, unfortunately, widely accepted by others. Until recently, neurologists believed that brain cells died off without being replaced. Psychologists affirmed the supposition by maintaining that the ability to learn trudged steadfastly downward through the years. Of course, certain capabilities fall off as you approach 50. Memories of where you left the keys or parked the car mysteriously vanish. Words suddenly go into hiding as you struggle to remember the guy, you know, in that movie, what was it called? And calculating the tip on your dinner check seems to take longer than it used to. Yet it is also true that there is no preordained march toward senescence. Some people are much better than their peers at delaying age-related declines in memory and calculating speed. What researchers want to know is why. Why does your 70-year-old neighbor score half her age on a memory test, while you, at 40, have the memory of a senior citizen? If investigators could better detect what protects one person’s mental strengths or chips away at another’s, then perhaps they could devise a program to halt or reverse decline and even shore up improvements. As it turns out, one essential element of mental fitness has already been identified. “Education seems to be an elixir that can bring us a healthy body and mind throughout adulthood and even a longer life,” says Margie E. Lachman, a psychologist at Brandeis University who specializes in aging. For those in midlife and beyond, a college degree appears to slow the brain’s aging process by up to a decade, adding a new twist to the cost-benefit analysis of higher education — for young students as well as those thinking about returning to school. Dr. Lachman is one of the principal investigators for what could be considered the Manhattan Project of middle age, an enormous study titled Midlife in the United States, or Midus. This continuing examination of Americans’ physical and emotional health and habits gained momentum in the 1990s as the first wave of baby boomers were settling into their fifth decade and running up against their own biases against aging. More than 7,000 people 25 to 74 years old were drafted to participate so that middle-agers could be compared with those younger and older. And with a new $21 million grant from the National Institute on Aging, the Midus team is beginning its third round of research this month. What makes Midus particularly valuable is that researchers can track the same person over a long period, comparing the older self with the younger self to see which capabilities are declining and which are improving. This approach has opened a new peephole into the middle-age brain. DESPITE continuing emphasis on SAT-type testing, in recent decades researchers have become much more aware of the range of abilities that constitute intellectual muscle. The Harvard psychologist Howard Gardner called his version of this theory “multiple intelligences” in his seminal 1983 book, “Frames of Mind.” “The human mind,” he later explained, “is better thought of as a series of relatively separate faculties, with only loose and non-predictable relations with one another, than as a single, all-purpose machine that performs steadily at a certain horsepower, independent of content and context.” Many researchers believe that human intelligence or brainpower consists of dozens of assorted cognitive skills, which they commonly divide into two categories. One bunch falls under the heading “fluid intelligence,” the abilities that produce solutions not based on experience, like pattern recognition, working memory and abstract thinking, the kind of intelligence tested on I.Q. examinations. These abilities tend to peak in one’s 20s. “Crystallized intelligence,” by contrast, generally refers to skills that are acquired through experience and education, like verbal ability, inductive reasoning and judgment. While fluid intelligence is often considered largely a product of genetics, crystallized intelligence is much more dependent on a bouquet of influences, including personality, motivation, opportunity and culture.