There are two major theories on aging. People who follow the program theory think that aging is based on a biological timetable. Others who believe in the damage or error theory say that aging occurs as the environment assaults the body and causes it to age. Many people believe it is a combination of biology and environment that causes aging. Organ systems in each person can age at different rates.
Hormones act as a biological clock controlling the pace of aging.
The immune system breaks down, eventually leaving a person more and more vulnerable to infection and disease. If a person has a high metabolic rate, his or her life span will be shorter. Proteins in the body accumulate. This slows down the body's processes and the body begins to age. Oxygen radicals accumulate in the body and damage it.