New Delhi, June 20 -- Ask most people what affects fertility as we age, and the conversation almost always turns to women. Male fertility gets far less attention, even though it is shaped by age, overall health and daily habits in equally significant ways. Men keep producing sperm throughout their lives, but the quality of that sperm does not stay fixed. It shifts over time, and fertility doctors are now seeing more young men dealing with low sperm count than they used to.

Where does age fit in?

The years between 20 and 35 are generally when male fertility is at its peak, with sperm count, motility, and overall quality typically at their best. "Fatherhood remains possible well beyond this window, but small declines tend to start after 3...