There are many theories and contributing factors, but, most importantly, when a cell divides by mitosis, regions of the chromosomes called telomeres are lost if the telomerase gene is not activated, which plugs and repairs these telomeres (which is why cells have a Hayflick limit (can only divide 50 times)). Now, often, as 99% of DNA or there abouts is responsible for only controlling and regulating protein synthesis, plus the activity varies from cell to cell, this doesn’t cause too many problems at first. However, as you become senile, middle-aged and decrepid like myself, and end up with about 100 trillion cells, this causes biological ageing and cell malfunction, because so many telomeres in so many places have been lost.
However, in people with cancer, the gene that activates the synthesis of the enzyme telomerase (a huge protein complex that adds the sequence TTAGGG to telomeres to repair them), which causes tumor cells to not have a Hayflick limit and continuously divide, so, actually, it’s better to be senile and old than have cancer with bulges popping out everywhere (no innuendo intended).
So, I hope I have summarised biological ageing for you!