"The biological clock for men and women is really the same," says Dr. Dolores Malaspina

According to a study released last March in the Public Library of Science Medicine, children born to fathers who were 20 scored an average of 2 points higher on an IQ test than children born to 50-year-old fathers. And that's not all. Recent studies from Israel, California and Sweden have connected "late paternal age" with any number of serious medical conditions: The longer you wait, the more likely it is that your kid will be affected by schizophrenia, dwarfism, bipolar disorder, autism, Marfan syndrome, certain childhood cancers, or even, later in life, Alzheimer's. In some cases, the risk factors skyrocket. A 2005 study conducted by the University of California, Los Angeles, found a fourfold rise in Down syndrome among babies born to men 50 and older. Worse still, those risk factors aren't limited to your tweed-sporting years: Statistically, "late paternal age" starts at 30, as in Zack's age. A 2006 study conducted by Mount Sinai School of Medicine found that fathers in their 30s have children with about 1.5 times the risk of developing autism compared with fathers in their teens and 20s. That factor jumps to five times for dads in their 40s. The cherry on the cake? The American Society for Reproductive Medicine recommends that sperm banks do not accept specimens from men over 40.

"The biological clock for men and women is really the same," says Dr. Dolores Malaspina of Bellevue Hospital Center in New York City and New York University, who conducted one of the first studies. "It's just that men can keep having babies."