Tick, tock, goes the male biological clock. According to the journal of Human Reproduction, a new study suggests that a man’s fertility starts to slide as early as his twenties.

A pool of 97 healthy, nonsmoking males between the ages of 22 and 80 were tested by Brenda Eskenazi at the University of California at Berkley and Andrew J. Wyrobek of the Lawrence Livermore National Laboratory. The lab results indicated that “semen volume waned each year and motility also declined significantly with age.” Eskenazi points out that “unlike women, there appears to be no evidence of an age threshold, but rather a gradual change over time.”