The breakdown of telomerase when DNA is replicated from a parent (old) cell to a daughter (new) cell. Telomerase is a protein that serves as a protector to the information in the DNA molecule, and is also responsible for the maintenance of cell health.
The upside is, people don't start to age until this protein begins to fail at protecting its DNA molecule, and faulty DNA is replicated. This is why babies do not have poor skin or faulty complexions, and why they do not suffer such superficial things as WRINKLES. (This is not including genetic abnormalities, which can't be compensated for unless damaged DNA actually fixes the problem....)