American fathers keep getting older, raising the prospect of increased birth defects but also greater economic and emotional security for U.S. families, according to new research from Stanford University’s School of Medicine.

The average age of the fathers of newborns in the United States has climbed by 3.5 years over the past four decades, growing from 27.4 years in 1972 to 30.9 years in 2015, said the study — the nation’s most detailed analysis ever of paternal age.

The number of newborns whose fathers were over age 40 has more than doubled over the past four decades. Those births now make up nearly 9 percent of births in the U.S., Dr. Michael Eisenberg and Yash Khandwala reported in the journal Human Reproduction.

The share of fathers who were over age 50 rose from 0.5 percent to 0.9 percent.

Similar trends are well-established in women. But while some celebrity fathers get headlines — actor Steve Martin became a first-time dad at age 67 and Mick Jagger of the Rolling Stones just welcomed his eighth child at the age of 73 — the trend among dads have received less study.

There’s no expiration date for men fathering a child. Unlike women, men don’t hit menopause — the time in a woman’s life when her fertility ends — so they can continue to have children into their later years. The oldest dad recorded in the Stanford study was 88 years old.

Because the average age of mothers of newborns has been edging up even faster than that of fathers, the gap between them has been shrinking, from 2.7 years in 1972 to 2.3 years in 2015, the study found. Women are delaying pregnancy due to educational and career goals, as well as greater access to contraception.

The Stanford researchers conducted a computational analysis of nearly 170 million births over four decades from the National Vital Statistics System, an intergovernmental data-sharing program sponsored by the federal Centers for Disease Control and Prevention.

Previous research has noted that men are delaying childbirth until later in life. But that research took a more general look at the data — lumping men into general age brackets, rather than identifying each father by age — to arrive at statistical averages. The Stanford study is also the first analysis of paternal age by racial and ethnic groups, as well as level of education.

“Our findings substantiate the need for further research on the health and social implications of older fathers,” said Eisenberg, director of male reproductive medicine and surgery at Stanford.

Asian-American fathers — men of Japanese and Vietnamese descent, in particular — are the oldest, becoming fathers at the average age of 36 years, the study said. Black and Hispanic men are the youngest fathers — age 30.4 and 30, respectively. White men, on average, have children at age 31.

Paternal age rose with educational attainment. The typical newborn’s father with a college degree is 33.3 years old — compared with 29.8 years for high school graduates.

But late fatherhood has been linked to higher rates of chromosomal abnormalities, neurocognitive disorders and spontaneous abortions, he said. It’s estimated that the male germline, which creates sperm, develops two new mutations every year.

But older fathers are also more likely to have better jobs and more resources. That makes them more likely to have stable lifestyles and more likely to live with their children and, thus, be more involved in child-rearing, he said.

Bay Area fathers describe both the joys and challenges of later-in-life fatherhood.

“I think the upside to being an older father is that I have more wisdom about life’s ups and downs. I’m more sanguine about the sacrifices it takes to raise kids, and I think I’m able to offer them a broader view of life than I might have been able to when I was younger,” said Alameda resident Paul Kotapish, who studied architecture at UC Berkeley and is now a professional musician with the band Wake The Dead. “We can offer a very stable home scene that isn’t rocked by the turmoil and changes that often accompany career building.

“The downside is, of course, that I don’t have nearly the energy and stamina I did when I was younger. Those years without much sleep really took their toll,” said Kotapish, who has played in remote fishing villages in Alaska as well as the Kremlin. “I don’t have as much vim and vigor for playing and horsing around as I’d like, and I occasionally crave more peace and quiet than is afforded when the kids have a house full of kids over and everyone is going nuts.”

Former Mercury News reporter Bruce Newman, a Santa Clara resident who had his first child at age 59, joked: “It’s nice to finally be part of the 1 percent.”

But, Newman said, he didn’t envy the men who become fathers in their 70s and 80s.

“Unless you’re (U.S. Secretary of the Treasury) Steve Mnuchin, whose IRA is going last that long?” he said.

William Phelps of Palo Alto, who was 55 when his son was born, cherishes the free time he has after a long career as an audiovisual consultant and systems programmer with IBM.

“I wasn’t a brand new employee needing to prove myself to anyone,” he said. “So I walked Byron to school every morning for 6 years, from kindergarten through 5th grade. I can still remember walking down our long driveway, turning the corner out onto the street, and his hand reaching up and taking mine. I got involved. I drove on field trips. I volunteered in the classroom. I knew every one of his teachers and the school principal.

“With age comes experience. We learn more about what matters to us. Our priorities become different, clearer,” Phelps added. “Now when he’s 15 we have conversations about things at so many levels, from science to relationships. We still hang out and do things. We pass books back and forth. I have to believe that some of the reason we have the relationship we do is because of the time I got to spend with him when he was younger.”

The advancing age of parents leaves fewer years for childbearing, so it will likely reduce the average family size over the long haul, with potentially huge economic ramifications, Eisenberg said.

“Fewer people being born means fewer productive workers a generation down the road,” he said. “This can obviously have profound tax and economic implications.”