When cells replicate, there is the occasional transcription error. As a person ages, these transcription errors accumulate. The reason that a lot of the elderly develop cancer, is the DNA governing when to replicate, and when not to, becomes corrupted. Some cell continues replicating past when it would normally stop.
Suppose that we compare a 60 year old man impregnating a younger woman to a 30 year old man impregnating her. In the case of the 60 year old father, the increase in risk of the child developing cancer is not nearly their father's. This is despite the fact that the DNA in the father’s cells is much more corrupted than the DNA in the 30 year old man’s cells. If sex cells degraded as readily as other cells, then the human race might go extinct after a few generations due to genetic defects.
What protects sex cells (sperm) from experiencing the same degradation as other cells in the body?