What causes aging?

Healthy Aging Image Gallery Aging is influenced by environment, genes and lifestyle. See more healthy aging pictures to learn more.

There are few physical differences among a group of first graders. But if you check out the same group 65 years later, their physical differences outnumber their similarities. Some will be the epitome of health, while others will be managing one or more chronic conditions. Some will be vigorous, while others will be lethargic.

As we get older, we become physically less like our peers. That's because we are the sum of our life experiences. At age six, not too much has happened to our bodies to make us radically different from our peers. But by middle and old age, we've had decades to develop and maintain habits that have an impact on our health, both negatively and positively.

The environment, too, affects our health, including where we work and live and how much exposure we have to infectious diseases. Aging is universal, but each of us experiences it in different ways.

­Aging may be inevitable, but the rate of aging is not. Why and how our bodies age is still largely a mystery, although we are learning more and more each year. Scientists do maintain, however, that chronological age has little bearing on biological age. The number of candles on your birthday cake merely serves as a marker of time; it says little about your health.

But which affects us more - our genes or our lifestyles? Find out on the next page.