Rupert Murdoch had a daughter when he was 72. Actor Tony Randall became a dad for the first time at 77. When the average life expectancy of the American male was a few months shy of 78, Nobel Prize-winning writer Saul Bellow fathered a kid at 84.

Long after a woman's biological clock stops ticking, most men can still father children. Yet many men say it's not just women who worry that they are too old to have kids. The physiology might allow for septuagenarians to bounce their beloved bundles on their arthritic knees, but the psychology suggests there is an age to stop bringing another baby on board.

Men having children past 40 is generally not a good idea, says Chris Mason, 46, of Danville. The father of three daughters by the time he was in his 30s, Mason says that he wouldn't consider having a fourth child, even if something were to happen to his wife.

"When your kids are young, you want to be out on the soccer field running, actually practicing with them," says Mason, the co-owner of a firm that out-sources sales. "But you get to a point where you can't keep up with the younger kids."

While the jury is still out on many details of the male biological clock, there is no consensus on the cutoff age for men to have kids, says Dr. Paul Turek, director of the Male Reproductive Health Center at the University of California, San Francisco.

"That's a fuzzy one," Turek says, "There is some evidence that as men age, their semen quality may decline
slowly, but only at 1 percent a year after age 40. It's really hard to draw

a line at some age."

Even though it's physically possible, are children being shortchanged by a dad bent on having kids so late in life?

"After 45 to 50, it's getting a bit late" to have children, says Simran Sandhar, 19, an aviation and business student at San Jose State University, who's not yet a father. "Any older, and the kids could be 10 or 20 when the father dies."

Ron Geraci, 36, author of "The Bachelor Chronicles" (Kensington Books, $14), says a responsible father should stop to consider the impact of a substantial age difference. The writer, whose father had him at 56, explains that his dad's deteriorating health often prevented him from playing a more active role in parenting. His father died in 1997, when Geraci was 27.

"It's too late to have a kid when you cannot guarantee that you're going to be in the game as a father when that kid is 18 years old," Geraci says.

For many men looking to father children beyond their 40s, having kids is often less a biological question and more of an individual, very subjective choice.

But just because a man can procreate in the winter of his years doesn't mean it's a good idea, says Jamin Favela, 24, of Hayward. He suggested men have their children by age 40.

"You want to watch your kids grow up," says Favela, the general manger at Camera Cinemas 12 in San Jose. "You want to help them figure out their lives. You really can't do that if you're an incontinent 80 year old."

Sometimes genetics, a healthy lifestyle or simple luck can allow older men to overlook their own chronological age and decide to become dads. Men such as Gregory, a Monterey Bay area antique dealer, who fathered twin daughters when he was 60.

Gregory, who asked that his real name not be used, already had children from his first marriage when he remarried a woman 24 years his junior. His kids from his first marriage are now in their 20s. Gregory had had a vasectomy and didn't plan to have additional children, but his new wife, eager to start a family with him, changed his mind.

Men should have children no later than when they're "40ish because you want to be at an age where you can enjoy things with your kids," he says of his twins who are now 6. "But I had kids when I was 60 because I'm a highly energetic, motivated and talented young guy!"

Dr. David Nudell, a urologist in San Jose, says that while men are capable of fathering children through their 50s, their sperm does become less effective at fertilizing an egg as they age.

"Women take fertility on their shoulders in this society and can feel inadequate if they don't get pregnant," Nudell says. Men have the same desires to have kids "but they hide it. I've had grown men just absolutely lose it in my office."

Regardless of how a man feels about his virility, a prospective dad should stop to consider how his advanced age will affect the life of the child, experts say. Ultimately, having a baby in life's late innings is a very subjective matter that can depend on such factors as health, financial well-being and the wife, Geraci says. But many dads bent on having kids aren't always weighing what's best for their children.

"They are selfish," he says. "You have to think about whether you want be a sperm donor or you want to be a father.

Fast facts about the male biological clock

Men over 35 are twice as likely to be infertile as men younger than 25.

Men interested in becoming dads later in life should avoid prolonged heat to their testicles, wearing briefs and sitting in baths or hot tubs.

Don't even think about lighting up; smoking hurts sperm's ability to fertilize an egg.

Exercise and a healthy diet -- including such supplements as vitamin E, zinc and folic acid -- can increase chances of conception.

''Avoid anything that has testosterone,'' urologist David Nudell says. ''I have seen family practice and holistic doctors who treat men with testosterone for energy problems.'' Anabolic steroids are also bad for sperm.

The risk of dwarfism, Down's syndrome and other genetic abnormalities in children increases for fathers who put off parenthood.

Sources: The Proceedings of the National Academy of Sciences; ''The Male Biological Clock: The Startling News About Aging, Sexuality, and Fertility in Men,'' by Harry Fisch, with Stephen Braun; Dr. David Nudell, urologist, of San Jose.