Plastic surgery can disguise the effects of ageing, but there is nothing that can slow the inevitable process towards death.

The fact that we age is no mystery, but there is no consensus on why human beings live so much longer than other mammals.

At the start of the century, people believed that the length of your life was determined by how fast your heart beat. So, the story went, hares lived fast and died young, while tortoises went on for ever.

Although we no longer follow this theory, we have not come up with a convincing answer to why we live so long.

Bud and Viola

The sixth episode of the BBC's flagship Human Body series follows seventy-eight-year-old Bud Mather and his wife Viola, 63, live in America's mid-West. Bud is a cattle rancher and has kept himself in good shape.

But Viola has watched his skin get rougher, his head get balder and his stomach get larger. For her, one of the first real signs of ageing was when she had to resort to glasses.

Robert Winston says eyesight is "arguably the most important sense". But much of what we actually see is an illusion.

The retina sees an upside down image which the brain corrects. A minute section of the eye helps us to focus, but we can only see small sections at a time clearly.

However, our brains persuade us that we are seeing a wider picture by piecing together different focused segments.

As we get older, the brain has more and more work to do to focus. When we are young, the eye's lens is pale blue. As we age, it turns yellow and by old age, it can have a brown tint.

However, we are not aware of the different tinge with which we are seeing things. The lens also clouds over and, in some cases, forms cataracts.

Monet

Impressionist Claude Monet gave the clearest insight into how ageing affects the eye. His vision gradually worsened and he eventually developed cataracts on both eyes.

His paintings became more and more cloudy and developed a red tinge.

After a cataract operation on one eye, he noticed the effect his problem had on his painting and threatened to destroy some of his earlier work.

Wrinkled skin is another major sign of old age. Exposure to sunlight and smoking are thought to accelerate the process.

Old people have thinner, less elastic skin than young people because the skin loses the proteins collagen and elastin over time. And the continual flexing of facial muscles leaves deep lines on the face.

Baldness

Baldness is another treat in store for the elderly. Men's hair thins before women's because of the ebb and flow of the hormone testosterone.

Women's hair becomes more sparse after the menopause when they lose the female hormone oestrogen and gain a little testosterone.

However, men's hair does not just disappear. It comes back with a vengeance in the ears and nose. And the nose and ears appear to keep growing in old age, although this may be more to do with sagging than real growth.

Bad news

More bad news comes in the form of lost hearing. As the years pass, the minute hairs in our ears which vibrate to produce sound die off. Sadly, we reach our hearing peak at 10 and it is downhill after that.

The loss of our sensory alertness makes it harder for our brains to function clearly which can lead to confusion and memory lapses.

And then there is the loss of physical strength and the onset of the medical problems associated with wear and tear.

By the age of 70, most people have lost a third of their optimum muscle strength, although, if they exercise regularly, this need not be the case.

Tide of fat

And then, to cap it all, comes the tide of fat which coats the stomachs of men and the hips of women.

Fat round the stomach is more easily absorbed into the bloodstream and can cause blocked arteries and heart disease.

But, paradoxically, old age is not just a story of decay. The body continues to renew itself. The lining of the gut, for example, is totally replaced every three days and the blood is renewed every three months.

Parts of the skeleton are reproduced every four years and most of our body is no more than 10 years old.

The reason we look old may be that the body is constantly copying itself and, eventually, the copies develop faults. Another theory is that free radicals in oxygen can damage the body.

It is thought that excess sunlight, air pollution and smoking may contain damaging free radicals. On the other hand, fresh fruit and vegetable and red wine may destroy them.

Pilot whale

Robert Winston says it was originally thought that humans lived to reproduce, but this would not explain the menopause. Only the pilot whale and humans go through the menopause.

Both spend many years looking after their grandchildren. Professor Winston says this function may have been overlooked.

And he says we should look on old age as our "greatest achievement" since human beings are the longest living mammals in the world.

As Time Goes By is the sixth programme in The Human Body series. It goes out on BBC1 on 24 June at 10.20pm BST. The series is accompanied by a £19.99 book and a CD Rom, priced £29.99.