A Man's Shelf Life

Most men can steer a gentler course just by watching their health. "Despite the research, there's still a big difference between the female and the male biological clock," says Muller. "When the female's alarm goes off at the end, that's it. For men, the battery slowly winds down. Yes, chance of problems increase as the years pass, but some men have significant DNA damage at 35, while others go on forever—their sperm is fine in their 70s."

Men can't rewind their biological clocks, but they can slow them down, Fisch agrees. Just remember, once you're in your 40s, you're past your maintenance-free years—you have to take care of yourself. "If you want children from then on," he advises, "get into the best shape of your life."