There are two very fundamental reasons why we age. The first reason is called "entropy". This is the tendency for all things to go from order to disorder. Your body is always making new cells, new skin, and new hair even though your hair eventually falls out as you brush it, or your skin becomes dry and flakes off. Your body is always maintaining order; in this way it rejuvenates itself. But because order always goes to disorder, eventually, as you age, your body is less able to do these things well. You begin to age.

The second reason is that everything exists at the expense of something else. This means that by existing, you use energy that other people, plants, and animals need also. Today you probably ate some plants and some animals. If you did not, those plants and animals would still be alive. If we all lived forever, there would be so many of us that we would use up everything. By aging and dying, we give others, including plants and animals, and our own children, a chance to live also.

Our bodies age and grow older, and this is a very natural part of the cycle of our lives. All living organisms, as they grow older, become more feeble and fragile, and their bodies do not function as well as they used to when they were younger.
Scientists have several different ideas about why aging happens to our bodies. One idea is that radiation (sunlight) causes damage to our bodies chemically and injures our cells. Have you ever seen carpet that has been bleached out by the sun? The colors of the carpet have faded because of interactions between the chemicals and the sunlight (photochemical or photo oxidative interactions). In reference to our bodies, the sunlight interacting with our body can produce harmful chemicals. Our bodies are dynamic organisms that can repair cell damage from these chemicals, but there are accumulations of these damages over time that our bodies defenses cannot overcome. The effects of these cell damages start to show after a certain time period (wrinkles on our faces and skin, an increase in the number of freckles and moles on our bodies as we become adults, cataracts in eyes that is common among elderly people, a weakening of bones called osteoporosis, cancer, a thinning of our stomach lining that protects us from our own digestive acid...). Some scientists also believe that your
diet (all that food you eat!) can influence how fast you age. It appears that lower calorie foods allow for a longer life than higher calorie diets.
This is probably because rich foods (high calorie) can produce harmful chemicals as they are digested that our bodies must break down.
Did you know that even respiration (breathing) can harm our bodies?
This is a very strange thing to think about because we must have oxygen for our bodies to function, but it is true! The element oxygen can have different forms in our bodes. One of these forms is the very reactive form called "oxygen radical" (peroxides and superoxides), and it is a waste product of healthy cell activity. These "radicals" are very damaging to parts of your body's cells (disrupting proteins). These radicals can also damage the blueprints of our cells structure, DNA. These blueprints replicate themselves and pass information onto the next generation of cells. Our bodies can change these harmful oxygen radicals into harmless forms using a special chemical (the enzyme catalase), but once the damage has been made to the cell's DNA, then future generations of cells accumulate the harmful effects of these chemicals produced from using oxygen.
Overtime these effects cause more and more cell deaths. By the time we grow old, we have fewer muscle cells to help pump the blood through our heart, and we have fewer cells in our brains that help us make new memories, and that access older memories. Older people have a harder time moving around because there are fewer nerve cells to relay signals from the brain to the muscles. These nerve cells have slowly been damaged or killed from exposure to pollution, radiation, and perhaps even our diet. It is very important that we pay attention to our bodies, and keep them as safe as we can from sunburns, harmful chemicals, and foods that are bad for us.
Eat your vegetables, students!
Some scientists believe that no matter how carefully we guard ourselves against harmful environmental conditions, our organs are only able to survive for a certain number of divisions or generations of cells.After humans fulfill their evolutionary requirement (produce new little humans and raise them until they can live on their own!), they have completed their job as organisms that must pass on their genes (blueprints for our bodies) to their children. Some say that it is amazing that we live as long as we do (avg. 70 yrs) because most people have their children
around age 20-30 yrs.

ps -Theres a very good DISCOVERY CHANNEL program on the human body and aging that you might like to watch.