A baby born in the west today will more likely than not live to be 105, write Lynda Gratton and Andrew Scott of London Business School in their crucial new book, The 100-Year Life. That may sound like science fiction. In fact, it’s only cautiously optimistic. It’s what will happen if life expectancy continues to rise by two to three years a decade, its rate of the past two centuries. Some scientific optimists project steeper rises to come.

If turning 100 becomes normal, then the authors predict “a fundamental redesign of life”. This book shows what that might look like.

We currently live what Gratton and Scott call “the three-stage life”: education, career, then retirement. That will change. The book calculates that if today’s children want to retire on liveable pensions, they will need to work until about age 80. That would be a return to the past: in 1880, nearly half of 80-year-old Americans did some kind of work.

But few people will be able to bear the exhaustion and tedium of a 55-year career in a single sector. Anyway, technological changes would make their education obsolete long before they reached 80. The new life-path will therefore have more than three stages. Many people today are already shuffling in that direction.

Two new life-stages appeared in the past century: teenagers and retirees. Now another stage is emerging, say Gratton and Scott: the years from 18 to 30, which people increasingly spend transitioning from education to full-time work. Of course, many of today’s young have no choice: they simply cannot find good jobs. But the 18-to-30s have also been quickest to understand the gift of extra years, say the authors. Many young people are now consciously searching and experimenting, working out how they want to spend the next seven or so decades. They don’t want to let life just happen, as it tended to for previous generations. Today’s parents, who grew up expecting shorter lives, should stop grumbling that 18-to-30s won’t “commit”.

Already the young are studying longer. The authors predict that more will do two degrees: first a general undergraduate course, which teaches thinking skills with lifelong value, and then a more specific vocational degree that teaches a specific sector’s current needs. After studying, the young will spend time travelling, exploring different sectors, and assembling a “posse” of friends and acquaintances who can sustain them at work and outside for 70 years. Instead of building old-fashioned CVs, people will build reputations on social media.

Future careers will contain many transformations. Lives will have fourth, fifth, even sixth acts. People will have to make more choices: next year, should you work flat-out in your job, return to education to learn new skills, or transition to an entirely new sector? There will be time to achieve mastery in multiple domains. No longer will women be denied careers because they took five or 10 years out to raise kids. That will still leave them 50-plus working years. Older people, especially, will develop portfolio careers. The trick will be to keep finding work that robots cannot do.

And people will change their use of leisure. When you could expect a 40-year career followed by fat state or corporate pensions, you could spend your free time chilling and buying stuff. But the 100-year life requires more saving. You might also need to spend much of your non-working time reskilling or exercising to maintain your body and brain for those extra decades. If that doesn’t come naturally, the authors have a tip: picture your 80-year-old self sitting by your side, tut-tutting each time you scarf a doughnut or book an expensive holiday.

Longer life can come as a shock, especially to those of us in midlife. We started work thinking we’d be done by about 60, and dead at 75. But now my generation can expect to retire at perhaps 75, and live to 90. The thought of having to do your current job for another 30 years can be daunting. In any case, many of us will be pushed out in our fifties. Some people (not me obviously!) may also need second or third marriages to take them through to 90.

In the 100-year life, age groups will mix much more than they do now. There will be more old people taking undergraduate degrees, or doing junior jobs as they descend rather than climb the corporate ladder. Many kids will grow close to their great-grandparents.

Most of this is to the good. Crucially, most of the years of life gained in recent decades have been healthy ones. But the book warns that the 100-year life could become the preserve of the well-off. Already the rich outlive the poor, and they will be better equipped to reskill and change careers. Poor people could face 60 years of dead-end jobs in the gig economy, followed by death at 80 without a pension. A life like that, say the authors, is “nasty, brutish and long”.