Many people think only a woman’s age affects fertility — but scientists say fathers have their own ticking clock too.

New UK research reveals that as men age, their sperm can develop harmful genetic mutations that increase the risk of passing diseases to their children. In men in their early 30s, about one in 50 sperm carried such mutations. By their 40s and beyond, it rose to one in 20.

Dr. Matthew Neville from the Wellcome Sanger Institute said, “We were surprised how strongly age drives up sperm mutations.” His colleague, Professor Matt Hurles, added that older fathers may unknowingly pass on “hidden genetic risks.” These mutations have been linked to autism and some cancers, though not every mutation leads to illness or pregnancy complications.

The findings come as sperm counts worldwide continue to fall — dropping nearly 60% in one generation. It’s a quiet warning for men: fertility isn’t endless, and time matters for fathers too.

Because sometimes, science reminds us — age changes more than just the face in the mirror.