Telomeres are DNA repeats at the ends of chromosomes. Their function is to maintain the integrity of chromosomes. Telomeres are shortened every time DNA is copied. Once telomeres are exhausted, chromosomes are prone to mutation, leading to arteriosclerosis or cancer. 
Experiments show that telomere length is correlated with aging and related diseases in people over 60 years old, but not in young people
 some people speculate that the length of telomere DNA can be used to calculate people's life span.