You age. As you age, your cells age. Your telomeres wear out. Even single-celled yeast age, giving birth to only a limited number of daughter cells. So how is it that each new generation starts fresh, unaged?
This is a great mystery - part of which is explained by the fact that your germ cells (which later turn into sperm and egg), those cells that will produce the next generation, are preserved with extra special care right from the start of your embryonic development. The non-germ cells, called somatic cells, make up all of the rest of you, and they age.
Stay tuned for a longer discussion of this here at Adaptive Complexity, but the latest news is that researchers have managed to give somatic cells some of the age-less character of germ cells, in worms (which are a great model organism for aging research).
We'll dissect the details here, but in the mean time, Nicolas Wade has a nice piece about the research in the NY Times:
A little piece of the germline’s immortality, it now seems, can be acquired by the ordinary cells of the body, and used to give the organism extra longevity.