"It wasn't all that long ago that any suggestion that a man had a 'biological clock' like a woman, and should father children sooner rather than later, would have been given short scientific shrift," says a new article by U.S. News and World Report. "Not anymore. Today, a growing body of evidence suggests that as men get older, fertility can and does decline, while the chances of fathering a child with serious birth defects and medical problems increase."

The article sources Dr. Harry Fisch, author of the book The Male Biological Clock with the finding that after age 30, testosterone levels decline about 1 percent per year. Fisch doesn't come out and recommend an ideal age for men to start a family, but where men have a choice in the matter, Fisch suggests "the sooner, the better."