Next year, there will be more people on earth over the age of 65 than under the age of 5.

The diminishing ratio of old to young is posing big challenges for this and other countries.

In a new book, 'Extra Time: 10 Lessons for an Ageing Society', the journalist and former Head of the Number Ten Policy Unit, Camilla Cavendish told Scotland Tonight if we take an optimistic approach, individuals and nations could reap the benefits.

Here is an edited transcript of her interview.

Rona Dougall: Camilla it's a fascinating book, what made you write it?

Camilla Cavendish: Well I started writing it after my father died two years ago, and I started thinking about the way in that age had really affected him, and believing he was old, long before he was old, had actually started to shrink his life. And one of the things that he most missed after my mother left him was cats. And he wouldn't ever get cats after she took the cats in the divorce, and was afraid that he would die, and leave a cat homeless. And he actually ended up living 36 years in really mostly very good health, without cats. And I started thinking a lot about that, and why did he think he was old when he was 50, really, and then I started interviewing a lot of people who clearly are not old at all, at 60, 70, or even 80. And I think we have got this whole new period of what I'm calling extra time.

Rona: Extra time, this new extended middle age, and do you think because of that we need to change our perceptions about old age?

Camilla: I think we do, the Japanese who are the world's oldest society now, have a word for it. They call it young-old. They basically have a group of people from 60 to 75 or even older who just don't have many health problems, are quite capable of going on enjoying life, and in fact doing jobs. We've have got this idea that early retirement is good, that at 60 you need to down tools, and start tending your garden. We also think that dementia is inevitable, for example, when actually the rates of dementia are falling rather not rising. So there are an awful lot of things that we need to update.

Rona: Do you think that older people are valued enough in the UK?

Camilla: I must say that I don't. In Africa, there are some brilliant examples of older people's wisdom, and experience, being really valued. In Zimbabwe they found that grandmothers are brilliant at helping people avoid depression. And giving therapy and grandmothers are helping psychiatrists across Zimbabwe because they have empathy, and a calmness and wisdom that actually come from being older.

Rona: And there's a Pacific island, in which retirement doesn't even exist!

Camilla: Yes in Okinawa, then live to enormously old ages, and they also don't get ill very much, and so they tend to die quite quickly. They don't have that terribly long period of half twilight of senescence that we all fear. They have no word for retirement yeah, they just go on looking after grandchildren, or fishing, farming, and they're also very active. And I think exercise is a very big part of this. All the research suggests that if you keep active you can have a much better old age.

Rona: Is it possible do you think to transplant that idea to the UK?

Camilla: Yes totally, we already have lots of evidence that shows that people who eat better and exercise regularly do much better, and I think we need to take on junk food. Scotland has brought in minimum alcohol pricing which is fantastic, we've got to take on obesity and diabetes and all the chronic diseases that affect people really young and make people old before their time. We need to take that on. You write about improving life not prolonging it, and you mention exercise and nutrition.

Rona: What other things did you come across in your research for the book?

Camilla: One of the other things is social connection, and the Japanese have a word 'ikigai' which means really reason for being. It is a sense of purpose. That does keep people alive for longer, studies show that you live seven more years of a good healthy life if you feel you have meaning in your life, and you feel you have good relationships. And I think giving people chances to be involved in mentoring, or public service, volunteering, there are great examples all over the world, where people really making a big contribution into their 90s.

Rona: To enjoy this extra time, this extended middle age that we're talking about; do you need to be sufficiently well off to enjoy that do you think?

Camilla: There is definitely a difference between the richest third, and the poorest third. In terms of their own health, and that is really shocking. I mean the gap in life expectancy from Glasgow to the richest parts of the UK is now about 11 years. And the gap in healthy life expectancy is also big. So it does help to be rich, to have better health, but it absolutely doesn't mean that you can't eat better and exercise and maintain great relationships whatever your income.

Rona: If you had to pinpoint one thing, what do you think is the most crucial thing, keeping you young as you get older?

Camilla: I think it's curiosity about the world; I have interviewed loads of old people. I interviewed the oldest stewardess in the world who is America, flies for American Airlines. She is 82. She just remains curious about every singly passenger, and that is clearly what really keeps people going.

Rona: If more people are living longer in the UK, is it inevitable that we will have to look at taxation, and will retirement ages have to go up even higher than they already are?

Camilla: I think retirement ages will have to go up with life expectancy. I think we are going to have to look at the social contract, and make sure we don't have to tax the young to pay for the old. And will mean doing things like charging national insurance on pensioners who keep working beyond pension age, I'm afraid those things will have to happen. But the truth is we have one in four Brits already unretiring, they are going back to work, because they are bored, they want the cash, and they can. And that's actually really positive.

Rona: You write about a $1 billion bet made by two American experts on ageing, I thought this was absolutely fascinating, one has predicted that there will be a 150 year old human being by the time we get to 2050. Do you think that is possible Camilla, and do you really want to live to 150? I wouldn't!

Camilla: I have a chapter in the book when I interview biologists about the breakthroughs they are making, and there are some drugs coming to market which will bring prolonged life and healthy life. And there are people looking really seriously now at the genetic pathways that can prevent our DNA from deteriorating. I agree with you Rona, I don't want to live to 150. What I want to do is be able to live as well possible, and then check out as fast as possible. Some of these biologists think that they are about to find ways that we can do that.