Time has you by the balls, gentlemen!
Cut the DT Friday Freakout intro nonsense, and just read this New Scientist headline!
Basically the spermatogonium, the stem cells that produce sperm, mutate all the time. But there are some "selfish" mutations that can persist and propagate more quickly than the normal ones. This can increase the numbers of mutant sperm being produced with a tranmittable disorder, which can increase the odds of a mutant sperm fertilizing an egg, when you're older.
There is no age range given for when your balls go full mutant sperm, so I'll just say 30yo and let you read the study yourself. Have a great weekend!
Testicular time bomb: Older dads' mutant sperm [newscientist]