For Clay Shields, the realization came seven years ago, when he was 31 and visiting a Northern California beach.
Mesmerized, he watched a toddler run toward the ocean, then suddenly stop to pick up a handful of sand and look at a rock.
"It was so neat to watch him discover the world. I thought, 'Sand is cool! Rocks are cool!' It made me see things that I'd been used to all over again," says Shields, now a computer science professor from the District. He'd always figured he'd want kids someday, he says, but it was that moment "that vague notion became concrete."
Today, Shields is still single and childless, but "I'm pretty optimistic it will happen," he says of fatherhood. "There's a fear it won't, but I don't dwell on it."
Bad jokes and tired stereotypes abound about panicky "clock-ticker" women who are desperate to find mates and reproduce. Yet there's a growing awareness that as men increasingly start families later in life, they may be feeling the pressure, too.
Earlier this year, Columbia urologist Harry Fisch's book "The Male Biological Clock: The Startling News About Aging, Sexuality, and Fertility in Men" (Free Press) delivered a tough message to men who are putting off fatherhood. While the age of the female partner is still considered a major determinant in achieving biological parenthood, Fisch, who runs the Male Reproductive Center at Columbia University, found evidence that men 35 or older may be twice as likely to be infertile as their counterparts 25 and younger. The book also cited studies suggesting that, as men age, the genetic quality of their sperm decreases significantly; under certain conditions, the children of older men may have a higher risk of Down syndrome and schizophrenia and their partners may have an increased chance of miscarriage.
Reproductive scares aside, many men say they also feel a psychological time clock, which grows louder as they watch their friends and siblings start families.
Journalist Peter Hyman describes it as a "palpable emotional vacancy" in his recent essay collection, "The Reluctant Metrosexual: Dispatches From an Almost Hip Life" (Villard). He jokingly calls his own experience being "Maclarenized," referring to the pricey baby stroller, and describes the "sensation of longing one feels when one sees a beautiful young couple pushing one . . . on a Sunday afternoon."
The paternal urge isn't an acute pang, many men say, the way it's often described for women, but more of a nagging reminder they should go ahead and get on with it.
"It comes and goes," says Michael Thompson, 40, a widowed federal telecommunications employee from the District, who says he's felt a desire to be a dad more strongly in the last two years. "I have a lot of nieces and nephews. I spend so much time with them and you watch them grow up. And you start thinking about what you're missing."
Experts say the longing becomes more pronounced as men delay parenthood. And men are becoming fathers later than ever before. According to the National Center for Health Statistics, fathers age 30 to 34 accounted for 27 percent of all live births in 2002 -- 8 percent more than in 1970. For fathers older than 35, that number nearly doubled, from 14 to 26 percent.
Anecdotal evidence says many of these men are first-time dads. The parenting Web site Babycenter.com, which claims 3.5 million visitors a month, for example, keeps an ongoing poll of actual or expectant fathers. Of nearly 4,000 respondents, 29 percent say they became first-time fathers between 30 and 34, 14 percent were 35 to 39, and 8 percent were over 40.
So what are men waiting for? Like women, whose age at first birth is also rising, they're finishing their education and getting their careers in order first, says Sandra Hofferth, professor of family studies at the University of Maryland.
"Getting to the point of being able to afford a family takes longer. The white picket fence is a lot harder to achieve these days," she says. The median age at which men marry today is 27; for women it's 25. In 1970, by contrast, it was 23 for men and 21 for women. Couples typically started a family within a year of the wedding, she says.
Jeremy Schneider, a therapist based in New York who writes aboutfatherhood for several parenting Web sites, says that for men who wait, fatherhood is "not so much a time as a stage in your life."
"It's that extra level of maturity," he adds.
Of course, delaying fatherhood can also come with growing pains. After getting used to frequent restaurant meals and weekend getaways, an older dad may have a harder transition than someone younger, says Steve Dubin, who teaches a boot camp for new dads in Boston. Older men may also fret about throwing out their backs on the ball field or having to work way past retirement age to pay for college. And few want to emulate the late Tony Randall, who fathered his first child at age 77.
But there's evidence that being older can have its advantages. According to Dubin, these fathers likely have more financial stability, patience and appreciation for what a baby brings to one's life.
Howard Schiffer, author of "How to Be a Family -- the Operating Manual" (Heartful Loving Press), started his own family at age 40. "I wasn't pulled in 100 directions," he says of raising his three children. "I'm like, 'This is the only time I get to experience this, and I don't want to miss any of it.' "
Older men feel "a greater poignancy to the legacy of having a child to carry on their name, values and estate," says Dubin.
Henry Dean, 45, a childless Internet consultant in the District, agrees. At family gatherings, he says, he's always the one playing with his nephews.
"I'll be disappointed if I make it through life without offspring," says Dean, who is single. "Sometimes, it really gets me. I get really depressed. But you deal with what you got."
Since his grandfather died at age 99, he expects that if he does become a dad, he'll stick around to be one for a while.
"I'll be a perfect dad," he says. "If I'm a little older, oh well!"