This BBC report is fascinating. As you get older, the protective caps at the end of the chromosomes, called telomeres, generally shorten. When they get short enough, the chromosomes stop dividing and cells stop replacing themselves. Except in sperm. They lengthen as you get older, and that larger cap is passed on to every cell in your children’s body.
The upshot is that older sperm can create longer-lived children, apparently. This may be one of many mechanisms by which older parents (who must live in a safer world) give birth to children who are built to thrive on long-term abundance, whereas if life is nasty, brutish and short we are biologically primed to more ready to fight and to reproduce early.