Telomeres are sort of "caps" on the ends of DNA, which protect the DNA when cloned.

With each cloning of a chromosome, the telomeres shortens, until eventually the DNA itself decays, inhibiting cell replication. In younger people telomerase replaces the telomere, but this process decreases with age, due to the availability of telomerase. Telomeres are shortened as we age, but telomeres can also be shortened by stress, smoking, obesity, lack of exercise and a poor diet.