In this 2008 study of twins, British researchers looked at 52 pairs of identical and 15 pairs of fraternal twins and concluded that exercise, even after controlling for confounding variables of age, BMI, smoking, year of sampling, and socioeconomic status, was associated with telomere length preservation. The effect was 6 years of length for 100 minutes a week and 9 years of advantage for 3 hrs or more.
When we consider my stem cell theory of aging, we can understand how all the things that are good for you are actually working. They are activating telomerase and keeping the stem cells preserved. We see this effect with mediation and other behaviors and we know that longer telomeres, independent of everything we can think of, lowers mortality risk.
To read the study in its original form, go here: http://archinte.jamanetwork.com/article.aspx?articleid=413815