An upbeat look at middle age? Patricia Cohen had her work cut out for her. Maybe there’s a capitalist culture somewhere that doesn’t make a blazing fetish of youthfulness and keep it stoked with huge amounts of commercial and medical hot air. If so, it’s not the one we live in. Yet nothing in the sometimes pathetic struggle of Americans over, say, 50 to claim a respectable place in the public consciousness puts Cohen in the grumpy mood she could so rightly claim after absorbing some 200 years of the history of middle age. Instead, she’s delivered a lively, well-researched chronicle of the social and scientific forces that brought midlife America to its current befuddled state — better off by most measures than any previous generation, but miserably out of fashion.

Despite the fact that researchers have been studying middle age intensively for decades, the term itself seems to have no fixed definition. Nearly any span between 40 and dementia appears to qualify, depending in part on whether we’re talking about ourselves (“But I feel just the same as I did when I was 20”) or all those people who show up at our college reunions (“Everyone looks so old”). But surely the elasticity of the concept is its best feature. After all, not much follows middle age except joining the ranks of the ancients — or, as geriatric specialists now say, “the old old.” (Perhaps they don’t intend to evoke the drumbeats of doom with that repetition, but otherwise what does it mean? A life stage so nice they named it twice?)

Not only is middle age hard to pin down, but Cohen, a reporter for The New York Times, points out that it didn’t even exist as a category until about halfway through the 19th century. Before then, she writes, “age was not an essential ingredient of one’s identity.” Life had markers like marriage, parenthood and physical incapacity; one’s years per se were not the measure. This changed with industrialization and the growth of cities, when age became a useful means of segmenting the population. Children who earlier studied all together in a one-room schoolhouse, for instance, were divided into grades by age. Birthday greetings became popular. By 1900, when the census began asking Americans their exact date of birth — instead of packing people into 10-year compartments — we were a nation thoroughly aware of how old we were.

By that time, too, people were living longer. But Cohen argues that the increase in years wasn’t significant enough to account for the emergence of middle age as a life category. More pertinent was what was happening to women. By the turn of the 20th century, Cohen writes, “the typical mother had two or three children, less than half as many as her counterpart 100 years earlier.” After the youngest left home, she had 20 or so years, starting around age 50, in which she was free of the primary obligations that had defined female existence for thousands of years.