Exercise in moderation is good for you and will help keep you younger. In this study (Med Sci Sports Exerc. 2008 Oct;40(10):1764-71) they showed that people who did moderate exercise had higher telomerase activity and longer telomeres than those that did none or too much.
Some other interesting findings are that running appears to be the best exercise, aerobic capacity is highly correlated with longer telomere lengths, and that unless you overtrain, the muscle cells can recover telomere length even in old age. Finally, having previously been an elite athlete in your 20s does you no good but exercise in your 40s and above is definitely beneficial.