Just how old is your old man? Researchers at Stanford University in Palo Alto, California, have found that since 1972, the average age of new fathers in the United States has increased by 3.5 years, Ars Technica reports. The study, published 30 August in Human Reproduction, used a federal repository of nearly 170 million birth records to find that the average dad is now 30.9 years old at their child's birth—and that 9% of newborns’ fathers are at least 40 years old. The average age of mothers at childbirth also rose over the last 40 years, confirming prior studies on increasing maternal age. Though the study did not focus on the reasons why dads are getting older, it does conclude that more research is needed on the potential health and social implications of older fathers.