But though men may not think about their biological clocks, CBS News medical correspondent Dr. Jon LaPook reports, they should.
"There are some men whose biological clocks tick a lot faster and a lot louder than others," says Dr. Harry Fisch, the man who wrote the book about that clock — and what happens when the timing is off.
"A lot of people think as they get older that there is erectile dysfunction. But there's more than just that," Fisch says. "There's excessive weight gain, tiredness."
One of the factors determining the pace of the ticking is testosterone.
"Testosterone is the major male hormone," says Dr. Peter Schlegel of Weill-Cornell Medical College in New York.
That hormone helps regulate mood, sex drive, fertility and muscle mass. And over time, it drops — at the rate of about one percent per year after age 40.
After age 60, roughly 20 percent of men are testosterone-deficient.
"One of the problems that occurs as men get older is they convert more testosterone to female hormones or estrogens," Schlegel says.
That's right. The structure of testosterone is remarkably similar to the structure of estrogen. With just one enzyme, the body can convert this male hormone into a female hormone — and body fat speeds up the process. So a bigger belly may mean lower testosterone.
But it isn't always that easy to spot.
Jeff Volk came to Schlegel with all of the symptoms of a man in a midlife crisis.
"I was lethargic and depressed," he says.
Sure enough, Volk didn't need a Porsche. A blood test revealed he needed more testosterone. He's been on hormone-replacement therapy for five years.
"It's increased the quality of my life," Volk says.
But testosterone is a powerful hormone that has many side effects and should be taken only under a doctor's supervision.
"We have not followed people for long enough to know testosterone replacement is completely safe, but it certainly appears to help with your cardiovascular health, with how you feel, with your bone density and a number of other factors, like how you think," Schlegel says.
So what's a guy to do to keep his testosterone levels normal, naturally?
"The obvious answer is that size really does matter — that is, the size of the belly," Fisch said. "The easiest thing you can do to keep your biological clock from ticking too fast is losing the weight around the belly."