Growing older
As you read this, your body is slowly changing. Cells in your bones and muscles are dividing. They are making your limbs, head and body larger. Cells in your skin are dividing too so your skin can grow to keep covering your body as it grows. It takes the cells about five days to grow and then divide. As soon as the cells have divided, they spend the next five days growing ready for their next division.

We are not really aware of these growth changes. Perhaps once or twice a year we may measure our height and be amazed at how we have grown. Sometimes people grow so fast that their trouser legs or jumper sleeves become too short in a few weeks.

If you look at a baby, you will see that its head is large compared to the rest of its body. In fact a quarter of the baby's height is due to the length of its head. As babies and children grow, the body and limbs grow faster than the head. By the time a person is fully grown, the length of the head only accounts for about a tenth of the height. If babies' heads grew at the same rate as the rest of their bodies, adults would have heads well over twice the size they are now.

People do not grow steadily throughout their lives. From birth until about the age of two years, there is a period of rapid growth. From the age of two until about eight years of age, the body grows slowly. This is then followed by another period of rapid growth often called the growth spurt. Girls begin their growth spurt before boys. They start growing rapidly at about ten years of age, a year or so before boys begin their growth spurt. Girls also finish their growth spurt earlier than boys do. This is why you may find that the girls in your class are taller than the boys. By the time you are sixteen, you will find that most of the boys are taller than the girls.

After the growth spurt, the body continues to grow more slowly until about the age of eighteen. After this, the body does not grow any taller although it may become heavier. In later life, from about sixty years of age, the body actually starts to decrease in height.
Other changes in the body also take place as a person gets older. Men, for example, may lose their hair. The hair of both men and women may turn grey. The skin, which has stretched and shrunk as a person has moved their muscles, becomes less elastic and forms wrinkles - especially on the face. Despite many changes in old age, old people can stay healthy if they take plenty of exercise and eat wisely.

Why do old people wear glasses when reading?
One of the body's changes that takes place with age takes place inside the eye. There is a lens in the eye like the lens in a magnifying glass. This lens is unusual because it can change its shape when the eye muscles pull on it. This changing in shape allows the eye to look clearly at near and far objects. As a person becomes old, the lenses and muscles become less able to work and so the person cannot see near objects clearly. For example, you may see an old person without glasses reading a newspaper by holding it well away from the face so they can see the words. When old people wear glasses, the lenses in the glasses compensate for the weakening of the lenses in the eyes, and books and papers can be seen clearly.

Why can't people live forever?
The cells, which make up all the organs of the body, simply wear out. This happens at different times for different people. If a person keeps a healthy life style of eating the right foods and taking exercise, they stand a better chance of living longer than people who have an unhealthy life style. Some activities, such as smoking, can shorten a person's life by causing heart disease and cancer.

Do people in different parts of the world live for the same length of time?
No. People in wealthy countries such as the USA, Canada, Australia, Japan and most countries in Europe generally live longer than people in poorer countries. This is because the people in wealthy countries can afford good food while people in poorer countries may not get enough food to stay healthy. People in poor countries do not have as many hospitals, doctors and nurses as people in wealthy countries, so if they do become ill there is a greater chance of them dying. Many organisations are working today to improve conditions in poorer countries so the people there can live longer and healthier lives.

How do people become smaller?
The main support of the body is the backbone. It determines the height of a person. The backbone is actually made from a number of bones stacked on top of each other. Between the bones are discs of cartilage. They stop the bones from rubbing on each other and allow the backbone to be flexible. As a person becomes old, the discs between the bones in the backbone shrink and this makes the backbone shorter so a person's height shrinks too.

Does the skeleton change in any other ways?
Yes, it does. Bones need a mineral called calcium to make them strong. In old age, the bones lose some of their calcium. This makes the bones become brittle so if an old person falls there is a greater risk of them breaking their bones. The places where bones meet are called joints. They make the skeleton flexible. In old age the joints may harden due to a disease called arthritis. This makes movement painful. In the skull the bones may become less angular and give the face a more rounded appearance. The bone in the lower jaw grows to make the jaw jut out and the bones in the cheek grow and make the cheeks hollow. In old people, especially old women, the hip joint may become badly damaged by a disease called osteoarthritis. This makes walking very painful and the top of the hipbone may be removed and replaced by an artificial one.