Forget those famous old dads of yore, including Saul Bellow (who became a father at 84), Charlie Chaplin (71), Pablo Picasso (68) and Abraham (100), immortalized in the Bible.
Age does make a difference for men who want to have babies, just as it does for women.
A growing body of research suggests that late-in-life dads are more likely to have fertility problems and are at higher risk of fathering children with conditions such as schizophrenia, autism and dwarfism.
And for the first time, sophisticated new scientific tests are suggesting a reason for the phenomenon: As men get older, the DNA in their sperm and its supporting protein scaffolding are more likely to break and suffer damage.
"For a long time, we labored under the assumption that sperm quality was universal throughout the life span of the male, but what we've found recently is that isn't so," said Dr. Richard Rawlins, director of the assisted reproduction laboratory at Rush University Medical Center.
"Instead, we now know sperm quality deteriorates over time."
In other words, men apparently also possess a "biological clock" for reproduction - albeit one that works differently than in women.
"We can't say there's a definite point before which (men) should try to have children or face the potential of an adverse outcome," said Brenda Eskenazi, a professor of epidemiology at the University of California at Berkeley.
"But we can say delaying fatherhood increases risk."
That assertion is controversial, as some doctors aren't convinced that the scientific evidence is conclusive or the potential harm is significant enough to cause substantial concern.
The risks are small, "the data is very limited and the pool of men potentially affected relatively restricted," said Dr. David Cohen, chief of reproductive endocrinology and infertility at the University of Chicago. "Very few 50-, 60- or 70-year-old men are trying to have a baby."