Telomeres are regions of repetitive DNA at the end of chromosomes, which protect the end of the chromosome from destruction. Their shortening in aging can block cell division, which may prevent instability and development of cancer in aged cells by limiting the number of cell divisions . Measuring their length can indicate the 'health' of the chromosomes they protect.

A study published in the Archives of Internal Medicine has examined the length of white blood cell telomeres in pairs of twins, to show the effect of different factors on cellular aging. Over 2,400 twin volunteers had blood samples taken and answered a questionnaire aboutphysical activity level, smoking status, and socieoeconomical factors.

A first analysis from all samples showed that white blood cell telomere length was positively associated with increased physical activity in leisure time. This association was not related to age, gender, BMI , smoking, socioeconomical status, or activity at work. To confirm that the effect was unrelated to genetic makeup, a small group of twin pairs who had differing levels of leisure physical activity were compared; their telomere lengths of the more active twin was significantly longer than that of the less active twin.

If that doesn't get you to the gym, I don't know what will. This is what the scientists say: "A sedentary lifestyle (in addition to smoking, high BMI , and low socioeconomic status) has an effect on leukocyte telomere length and may accelerate the aging process".