We live in an age of generational turmoil. Baby-boom parents are accused of clinging on to jobs and houses which they should be freeing up for their children. Twentysomethings who can’t afford to leave home and can’t get jobs are attacked as aimless and immature. Both sides of the generational divide should take comfort from this timely, thoughtful work by Steven Mintz, professor of history at the University of Texas at Austin. In Mintz’s view, no one is to blame for these changes, neither the selfish baby boomers nor their Peter Pan offspring. What is happening is a shift in the nature of adulthood, and to understand this we need a historical perspective.

To most of us, adulthood means being able to earn a living, possess a home, get married and rear children, and this implies having autonomy or control over one’s life. In the 19th century becoming an adult was celebrated as a liberation from paternal authority. Today we regard it more as a time of regret and stagnation. It isn’t cool to be adult. Mintz argues for a new understanding of adulthood. It should be seen less as a matter of a steady job and marriage, and more as ‘maturation’ or psychological development.

As Mintz shows, life stages such as adulthood are not fixed and permanent, but cultural and institutional constructs. Childhood was discovered as a distinct stage in the late 18th century, and it was institutionalised in the 19th century by age-segregated environments such as schools. Adolescence was only recognised as a life stage in the early 20th century, when psychologists got down to work.

Today’s generational battle obscures the fact that adulthood is happening later. A new transitional stage has emerged after adolescence: the twenties. Sexual initiation occurs earlier than in the 1950s or 1960s, but financial independence and career entry comes much later. The baby boomers were exceptional in getting jobs and leaving home in their early twenties — the result of the extraordinary economic opportunity of the 1960s. Today, the twenties have replaced the teens as the most risk-filled decade, when young people really do depend on the emotional and financial scaffolding provided by their parents. We criticise helicopter parents for infantilising their offspring by delaying the separation they need to grow up. According to Mintz, however, the young people who do best are the ones who postpone commitment and stay at home in their twenties.

Mintz is upbeat about scaremongers who worry that social networking is destroying people’s ability to interact. It is true that Facebook is no substitute for face-to-face contact. But deeper changes were at work in the 20th century which altered the nature of intimacy. Romantic love came under attack, first from the Freudians and then from the neuroscientists, who said that being in love was a chemical reaction in the brain. Marriage is no longer seen as a lifetime commitment. Nor does it lock couples into an inward-looking relationship where their only friend (or enemy) is their spouse. The result is a boom in friendship. More than ever before, people look to friends — especially of the opposite sex — for intimacy.

This is an American study, and there are obvious differences between the US and Britain. Americans are more likely, for example, to divorce than Brits, and also more likely to remarry — partly because the lack of a welfare state means that they depend much more on family for care. But in Britain, as in America, marriage is no longer seen as a gateway to adulthood. At least among the educated, it has become an optional extra, a ‘capstone’, confirming a live-in relationship rather than beginning one. Hence the fall in marriage rates — though, paradoxically, as heterosexuals retreat from marriage, gays and lesbians are seeking access to it.

Mintz claims that American parents are the most anxious in the world, but they don’t seem all that different from their British counterparts. Childhood has become fraught. Parents panic about cot death and autism, about television and online porn. The result is a generation of bubble-wrap kids, protected by parents who are overly risk-averse. It’s not the children who have changed but their parents. This, says Mintz, is partly down to scans in pregnancy, which were meant to reassure the parents but actually make the foetus the focus of anxiety. More important is the fact that middle-class families project adult expectations of success on their children.

Adulthood has become much more angst-ridden. But for Mintz all the stress and inability to cope is a positive thing. It’s the result of the breakdown of the old, restrictive idea of being grown up, and the shift to a new state of adulthood which, in Nietzsche’s words, is ‘a work of art that we ourselves create’.

As you might have gathered from this review, reading this book is hard work. It is densely packed with information and argument, and at times Mintz loses sight of the adulthood theme. But it is a great book, and a triumph of historical writing; it shows that the past really can explain the present.