When Anna Williams died in a nursing home in Wales in December of 1987 she was believed to be the oldest person on Earth. She was 114.

Scientists think that's about as old as it gets - 115 to 120 is probably the upper level of human longevity, they say. What they don't know is why. In fact, when it comes to aging, there is still a lot we don't understand.We know it happens to everyone - but we don't know why it happens differently in different people. Nor do we know how to do much to alter its course.

There are certain predictable changes, of course. Gray hair is one. So is wrinkled skin. Physical and mental vigor decline, and so do muscle and bone mass. The heart becomes less efficient; the kidneys may not work as well; eyesight diminishes.

But the first gray hairs appear for some people as early as their 20s; for other they don't come until the 40s. In some 70-year-olds, heart and kidneys function as well as in some 20-year-olds. As one scientist told FDA writer Ken Flieger, "knowing that a 75-year-old man has diabetes and a history of heart disease doesn't provide a clue as to whether he's sitting in a nursing home or on the bench of the Supreme Court."

Research is, however, starting to piece together some parts of the puzzle, and since we have just passed another corner in the calendar that means we are all a year older, this might be a good time for a status report.

There are many theories of aging, but they fall into two categories, says Flieger: Error and Program theories.

ERROR: The speculation here is that with advancing age we become less able to repair damage caused by internal malfunctions or external assaults to the body from such things as pollutants, viruses, carcinogens, solar radiation. Much like an automobile, the human machine can sustain only so much damage until effective repair is no longer possible. The body simply ceases to run.

PROGRAM: The other school of thought suggests that an internal clock starts ticking at conception and is programmed to run just so long and no longer, although everyone's clock is different. According to this theory, genes carry specific instructions that enable both growth and maturation, but decline and death.

Whatever theory or theories prove to be true in the long run, scientists think they have at this stage pinpointed where the action takes place: in the cells. Among the most popular Error ideas about what happens are:

- The wear-and-tear theory, which suggests that cells gradually lose the ability to repair damaged DNA, the substance that passes genetic information from one cell to the next. As this occurs, cells become less efficient in carrying out vital functions, such as making proteins, and they eventually die.

- The fast-living theory, which speculates that aging is related to metabolic rate, the rate at which cells convert nutrients into the energy they need to live and reproduce. The higher the metabolic rate, the faster a person lives and dies. This would explain why aging characteristics vary in individuals.

- Still another theory hooks aging to damage caused by protein "cross-linking." As cells age, more and more protein molecules in cells and tissues become chemically bound to one another in ways that interfere with theirnormal functioning.

The Program theories, on the other hand, view aging as a genetically determined program. Scientists have determined that cells, due apparently to some element in the genetic code, have a built-in capacity to divide only about 50 times. The only exception is abnormal cancer cells that go on dividing indefinitely.

But human cells divide at widely different rates. Nerve cells may never divide once nerve tissue has formed. Tumor cells, by contrast, may divide as often as once every 18 hours.

Scientists stress, however, that it oversimplifies things to think that

lifespan is entirely related to how many times a cell can divide; but they think there may be some relationship. Other factors, such as hormones, are likely involved.

Many of the answers are still out there, but as our population ages, aging has become a hot area of research. A wealth of information has been uncovered in the last couple of decades, and scientists are optimistic about future discoveries. Spearheaded by the National Institute on Aging, one of the National Institutes of Health, scientists will continue to probe the mysteries of this interesting phenomenon.

And in the meantime, is there anything you can do to minimize the impacts of aging. The National Institute on Aging says your chances of staying healthy and living a long time will improve if you:

- Don't smoke.

- Eat a balanced diet and maintain your desirable weight.

- Have regular health checkups, see a doctor when you detect a problem and follow a doctor's advice about taking medications.

- Stay involved with family and friends.

- Allow time for rest and relaxation.

- Get enough sleep (the amount needed varies from person to person and may change with age - you probably know your limits).

- Stay active through work, recreation and community activities.

- Drink alcoholic beverages in moderation, if at all, and don't drive after you drink.

- Use seatbelts.

- Avoid overexposure to the sun and cold.

- Practice good safety habits to guard against fire, falls and other accidents.