Telomeres are on the ends of every chromosome (strand of dna). They contain no genetic information themselves. Every time your cell splits, special enzymes attach themselves chromosome, copying everything as they go, including the telomere. However, the enzymes, due their shape, can't actually travel all the way down a dna strand; they don't actually copy the last little bit of dna. If there were genetic information on the end of the dna strand, it would be lost. However, there are telomeres which act as a protective buffer and it just so a little bit of the telomere is lost instead of genetic information. However, because a little bit of the telomere is lost with every split, eventually the telomere shrinks and vanishes. At that point, the cell can no longer copy itself without losing information, and so it doesn't.

This company seems to have found a new way to regrow the telomere in young cells.

http://en.wikipedia.org/wiki/Telomere