NEW YORK, Nov. 15, 2006

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(CBS) The biological clock has always been seen as a woman's problem, but mounting research shows the clock is ticking for men. While fertility doesn't decrease as dramatically as for a woman, a man does have a biological clock, experts say. But these scientists disagree on exactly when the alarm sounds.

How is men's fertility affected by age?

A man's fertility begins to decline in his 30's just as it does for women, then steadily drops through his 40's and 50's.

While a woman is born with all the eggs she will ever have, a man produces a hundred million sperm each day through cell division. The more often a cell divides, the more likely a glitch will occur as the genetic code is copied.

As Harry Fisch, M.D., from the College of Physicians and Surgeons at Columbia University tells CBS News, for statistical reasons, there is a huge difference between men in their 40's compared to men in their 20's, but men in their 30's are also at risk. For some disorders such as Schizophrenia, there is a steady rise in the incidence of that disorder.

But for disorders such as Down Syndrome and miscarriages, the fathers' negative contribution seems to affect only women in their mid-30s. This represents a combination problem or a combined parental age problem, where the ages of both parents matter, according to Fisch.

What are some possible risks?

A study published in 2001 in the Archives of General Psychiatry found the risk of schizophrenia in children was associated with older paternal age. For instance, children of fathers over 50 were almost three times more likely to have schizophrenia than children born to the youngest fathers, the research found. The database included nearly 90,000 people.

In another study, more than 3,400 cases of Down syndrome were studied. The researchers found that thee father's age played a significant role when both parents were over 35 at the time of conception. The effect was most pronounced when the woman was over 40. In those cases, the researchers "found the incidence of Down syndrome is related to sperm approximately 50 percent of the time." These findings appeared in the June 2003 issue of The Journal of Urology.

Can men slow the decline in fertility?

Fisch tells CBS News that when he speaks to young couples, he always encourages them to start planning a family sooner rather than later. However, when he counsels older couples, he stresses that current genetic testing of sperm in not accurate enough to determine if a problem will occur.

WebMD offers several recommendations for men wanting to maintain their maximum fertility. They include:

Maintain optimal weight, cut out recreational drugs, and stop smoking.

Taking care of seemingly unrelated conditions may help too. High cholesterol is one of them.

To learn more about male paternal age:

• You can read more from the Male Reproductive Center at Columbia University in New York City.

• Read more about men's biological clock at WebMD.

• Click here for information from the Society for Male Production and Urology.

• You can read more about biological clocks from the National Institute of Mental Health.