Those of us born soon after 1945 belong to the most undeservedly lucky generation in history. As children in the 1950s, we enjoyed the benefits of an affluence that spread around the world from the US; as adolescents in the 60s, we made love, not war, and defied a society whose restrictions irked us. When we chopped off our long hair and consented to join the workforce, we were guaranteed a job for life. That security set us free to transcendentally meditate or pump up our muscles at the gym, pursuing what came to be known as "personal growth". Unworldly idealists when young, we aged into an avaricious lot, encouraged during the 80s by venal gurus who licensed our vices by proclaiming that "greed is good".

Now, as we prepare to keep ourselves going with artificial hips, flesh-coloured hearing aids and sly doses of Viagra, our longevity is bankrupting the pension system and disinheriting our successors. As the American humorist PJ O'Rourke puts it in this book on our collective rampage: "We're riding down the highway of life in a Welfare Cadillac" – and almost incidentally, the emissions from that plush vehicle and our other gas-guzzling toys have poisoned the seething planet. With our obsessive self-consciousness and our querulous identity politics, we may have created "a personal universe", but we will leave behind us a depleted, debilitated Earth.

O'Rourke sums up our legacy in an acrid aphorism. The final subset of boomers, he claims, were the punks, while the goths, who came next, were the advance guard of those who will inherit a shrivelled and unwelcoming world. The first group shouted: "Fuck you", the second whimpered: "We're fucked"; the subtle shift, as O'Rourke says, marks an epochal transition and he therefore apologises to Generations X and Y for having obliged its members "to pierce their extremities and permanently ink their exposed flesh" as a morbid sign of despair.

O'Rourke dismisses our self-righteous claim to have effected a revolution during the 60s. That carnivalesque decade, he says, was merely "a comic interlude in a century that needed one". Anti-Vietnam protests gave us an excuse to mock authority and vandalise property; free love did not demilitarise the world and anyway our happy-go-lucky promiscuity was soon curtailed by herpes. "The Baby Boom's greatest achievement," O'Rourke concludes, "has been in the field of bullshit." We slickly perfected the arts of communication and specialise in mind-befuddling, manipulative professions such as public relations, political campaigning and high finance. (O'Rourke shows off his own expertise as a marketer by warning British readers that they won't understand his American frame of reference, then urging them to buy his book anyway.)

A demonstration outside the Pentagon against the Vietnam war in 1967. Photograph: P Conklin/TPX/Rex Features

Given the magnitude of the crimes with which he charges us, it's surprising that O'Rourke's tone is so relentlessly upbeat, with a joke in every sentence and a pause for guffaws at the end of most paragraphs. In the past, his comedy has seemed sour and bleak. His travel book Holidays in Hell paid perverse visits to the most wretched places on Earth, including Managua, Seoul, Beirut and (a little bathetically) Belfast; Bangladesh for him was indistinguishable from the 1969 pop concert at Woodstock – "overcrowded, muddy, lacking in food and public order". Now, after sampling hell, O'Rourke nostalgically journeys back to a remembered heaven. Boomers, having been the first teenagers, treat adult life as a prolongation of irresponsible adolescence; O'Rourke accordingly retreats into a cosy memoir about growing up in dull, bland, suburban Ohio.

Shelving the self-accusation that is supposed to be his book's thesis, he spends much of it rambling through this "good and happy place". He recalls games of hopscotch on the chalked pavement, lemonade stands in the street, freshly mown lawns and fences across which neighbours actually talked to each other. A little later, with the start of pubescent itches, he regales us with tales of exploratory petting in parked cars. That infantile paradise has never been definitively lost. "We were on probation from the Garden of Eden, not expelled," says O'Rourke when describing the petty misdemeanours he and his buddies committed; "We had a day pass to get by the cherubim at Eden's gate."

Why this idyllic detour, which exchanges satire for nostalgic sentimentality? Genius may be childhood recovered at will, but so is senility. But O'Rourke's regression softens and muffles his case against his contemporaries. His railing turns out to be mostly bluster; the real aim of his book is to exonerate the overgrown babies of the boom generation, to insist, as he says in his book's subtitle, which paraphrases the contradictory pleas of a child caught with their grubby fist in the cookie jar: "It wasn't my fault and I'll never do it again."

He begins sternly enough, with an adult sense of accountability, recognising that "the world is our fault". Then the defensive quibbling begins. After all, as he admits: "We are the generation that has an excuse for everything – one of our greatest contributions to modern life." We have invented syndromes to explain and justify our addiction and distempers; in an emergency, we can always plead that we are suffering from post-traumatic stress disorder, though the trauma may have been our tantrum when we realised that the world was not going to satisfy our demands.

Steubenville, Ohio, 1958. Photograph: Francis Miller/Time & Life Pictures/Getty Images

After shrewdly analysing this self-deception, O'Rourke goes on to exemplify it. He preposterously claims that our self-indulgence has been responsible for "the Earth's increase in widespread wellbeing", because war is just too much trouble for us – yet haven't we waged an annihilating war against nature itself? A similarly skewed argument pretends that we're harmless because power interests us less than money, so that the best and brightest in our "age cohort" are not in Washington, but on Wall Street. Isn't O'Rourke aware that bankers and financial speculators can do as much damage to society as the politicians who are their stooges?

Even wonkier is his contention that whereas the generation of our parents dropped atom bombs on Hiroshima and Nagasaki, boomers can take pride in having shown that Saddam Hussein didn't possess nuclear weapons. Does that make Saddam a pacifist? With the reasoning this tortuous, it's impossible to tell whether O'Rourke is joking or not. He may not know the difference, since his book is a jovial "celebration of the mess the Baby Boom has made": social malaise, economic collapse, political terror and ecological disaster are for him all equally hilarious.

This late in the day, the very word "boom" sounds hollow, like the echo of a blast; as we know only too well, it leads inevitably to bust and that cycle of expansion and implosion is replicated in O'Rourke's writing, which twists an impending catastrophe into a comic double take. Summing up, he says: "We are the best generation in history. Which goes to show history stinks." Even so, he can't stop at this dead end and adds a further laugh line as a meaningless reassurance: "But at least we are fabulous by historical standards." Fabulous or fatuous? Humour is balm and it washes away blame; at the same time, it trivialises O'Rourke's entire undertaking. Nero complacently chortles while Rome simmers, sizzles and starts to burn.