Karen had a birthday recently. She turned 10. At her party, after she blew out the 10 candles -- plus one to grow on -- she said, "I can't believe I'm already 10."

With each passing year, Karen's body has changed. She has grown from a tiny baby into a healthy child. Her body will continue to change as she grows into a teen-ager and then an adult. She will get taller, heavier and stronger. Eventually, she will stop growing -- and start aging.

How does Karen's body know how to grow bigger and stronger? Like you, she has a system of glands in her body that produce special substances called hormones. As Karen gets older, these hormones tell the cells that form her body to soak up lots of nutrients, or food. Using this nourishment, her bones will become longer and harder. Her muscles will grow, and she'll get stronger and more physically coordinated. Her body will change shape, too, as she makes the transition from childhood to adulthood.

By the time Karen is about 21, she'll be all grown up. All of the systems she needs to be a healthy adult will be in good working order. If she eats well, gets lots of rest and exercises, she'll probably feel terrific, and go on feeling that way for many, many years.

Eventually, Karen will begin to grow old. You probably know some elderly people. They may be members of your family, or friends. When you spend time with them, you probably notice that they move more slowly than younger people do. They may not hear or see as well. They may be more likely to catch illnesses than younger people are. These changes are all part of growing old.

If you know any older people well, you probably also know that even though they may not be able to move as fast as you can, or see as far, that they have a lot to offer you. For example, Karen has a great- grandmother who is over 80 years old. She came to Karen's birthday party recently. After the cake was all eaten, Karen's great-grandmother taught the guests a card game she had played when she was a little girl, and told them about the parties she used to have on the farm where she grew up.

The difference between the bodies of young people and old people has to do with their cells. A human body contains trillions of cells of many different kinds. Some form your bones. Cells are the basic building blocks that make up your blood, your muscles and your brain -- in fact, every part of you.

As you move and grow, your cells experience a lot of wear and tear. But that's okay, because they can repair themselves. As you get older, your cells are less active, and less able to protect themselves from the stresses of daily life. When Karen's great-grandmother's skin cells slowed down, her face grew wrinkled. As cells in her hair stopped producing pigment, or color, her hair turned gray.

Today, doctors have developed lots of ways to help aging people feel better and healthier in spite of their years. In the United States, people can expect to live healthy, productive lives for around 70 years -- and many Americans live even longer than that. Tips for Parents

Are you lucky enough to have an elderly family member who takes an active role in your children's lives? These relationships can be extremely important to children. But what happens when these vital people die?

While very young children think of death as a reversible condition, the American Academy of Child Psychiatry reports that between the ages of 5 and 9, kids begin to think as adults do about death. Even so, they are unlikely to believe it will happen to anyone they know.

When someone close to your child dies, the child may show little immediate grief, or may exhibit marked anger toward surviving relatives. These are normal childhood response, as is initial denial that the death has occurred. But long-term denial may be unhealthy. The following danger signals may indicate that a child is having a serious problem dealing with loss, and may need professional help to with the mourning process:

*An extended period of depression, in which the child loses interest in daily activities and events.

*Inability to sleep, loss of appetite, prolonged fear of being alone.

*Acting much younger for an extended period.

*Excessively imitating the dead person; repeated statements of wanting to join the dead person.

*Withdrawal from friends.

*Sharp drop in school performance or refusal to attend school.