Flexi Says: Measuring the length of telomeres is a classic method for estimating cell age. Telomeres are repetitive DNA sequences that protect the ends of the chromosomes from degradation. But every time cells divide, they lose a few DNA bases at the end. This is because the machinery of replication has to sit on the end of the chromosome to complete replication so the DNA under the machinery cannot be copied. Therefore, the telomeres get shorter each time a cell divides. Other methods for estimating cell age involve more complex analyses of cell metabolism and biophysical properties such as cell flexibility. As cells age, their metabolism changes and they become more rigid.