By Michael Nedelman

CNN

(CNN) -- Actor George Clooney, 55, will soon to be a father to twins with his wife, human rights lawyer Amal Clooney, 39 -- reigniting a conversation about what it means to be an older dad.

Despite growing awareness of in-vitro fertilization and aging mothers -- such as Janet Jackson, who gave birth last month at 50 -- some of the science and social impacts of aging fathers are less clear. But that isn't deterring hopeful dads from growing their families.

"It's very well-documented: Men are having children later into life," said Dr. Mary Samplaski, a male-fertility specialist at Keck Medicine of the University of Southern California. "It's become so much more mainstream."

Men's fertility does not drop as sharply as women's as they age, said Samplaski; many can have biological children long after they're old and gray without the need for technology to intervene. But some of Samplaski's patients have questions about the risks of fathering later in life.

Knowing the risks

A Swedish study in 2014 added to the evidence that dads older than 45 are more likely to have children with schizophrenia, autism and other psychiatric disorders. Still, even for these fathers, the absolute risk was found to be very low -- well below 1% for certain conditions.

For one man who saw the study, the findings pushed him into action.

"I reported on a big study that came out in 2014, and I thought a lot about it, and a few months later, I went and banked my own sperm," former CNN producer William Hudson said in August.

"I banked my sperm because I wanted to have the option of using younger sperm later in life."

To explain the link between mental illness and paternal age, researchers have pointed to the mutations that accumulate in sperm DNA as men get older, which may be inherited by their sons and daughters.

There are no genetic tests for psychiatric illnesses such as autism spectrum disorder and schizophrenia, Samplaski said.

However, last year, another study in Nature Genetics took an opposing view: Spontaneous mutations in sperm DNA were not largely to blame for the increased risk of psychiatric illness. The researchers found that genes might contribute to this risk, but only a small portion of it -- about 10% to 20%.

A closer look at the data shows that mental illness isn't just linked to older fathers; there's a risk among younger fathers, too. Some researchers say the focus on mutations could be a red herring. According to the authors of the Nature Genetics study, fathers with a heritable mental illness may simply be more likely to have children at the extremes of age -- perhaps because they socialize or take risks differently.

Though a similar trend has been found for IQ scores, other studies have refuted those results by taking into account paternal intelligence.

A number of other conditions have been linked to late fatherhood. One of the most common is a type of dwarfism called achondroplasia, for which there is a genetic test. For men over 50, the risk of achondroplasia is only slightly more common than 1 in 2,000 births.

Some doctors have proposed that aging sperm may come with a benefit: longer telomeres. Telomeres are the tips of chromosomes that protect DNA while it's being copied -- like the aglet at the end of a shoelace that prevents it from fraying. Many scientists believe that longer telomeres signify longevity. However, no strong data exist to suggest that children of older parents actually live longer.

Samplaski said the risks that concern her patients are not always genetic. For many, it's a simple numbers game.

"If you're 55 when you have this baby, you're going to be 65 when this kid is 10," she said, adding that people's numerical age may not always be the best predictor of their life expectancy.

"Most of the couples that are having babies later in life are taking care of themselves."

Too old for this?

"I got tons of blowback about it," said freelance writer and television producer Philip Lerman, who has written about his decision to become a father at 46.

After the Clooneys' news spread on social media, some on Twitter compared George Clooney to a grandfather. Lerman said the reactions he received mirrored that.

In one Humans of New York photo that drew more than 70,000 responses, a young woman with a 78-year-old father said that it's "hard to be this young and watch them go that way already."

While some children of older parents commented about the difficulties caring for sick and aging parents, others spoke of the valuable lessons they had learned from parents with more life experience.

Another story that drew strong responses was a 2011 New York Magazine cover story featuring a white-haired model who was digitally altered to look pregnant placed next to the headline, "Is she just too old for this?" In response to the article, which addressed both men and women, some readers called out aging parents for being "selfish," lamented not having grandparents in the picture and expressed worry about older parents' mortality.

Lerman, now 61, has a 14-year-old son named Max, whom he and his wife conceived using a donor egg. People told Lerman he was "too old" to have a child for many of these reasons. Some suggested that he would have trouble keeping up with a kid and that he might put his future son at risk for genetic diseases.

"I took a chance on having ... a child with problems," Lerman said, adding that it's important to think about the alternative.

"What if I had listened to them?" he added. "What if ... Max didn't exist, and he didn't get to play Little League, and he didn't get to be the star in the school play last year?"

Lerman admits that he "can't do the things that a younger man can" but said children like Max benefit from parents who have taken time to become more emotionally and financially stable. Where some have criticized his age, Lerman believes that older parents are just one manifestation of the diverse American family.

"Max has an older dad; his friend has two dads," Lerman said. "We've got adopted kids, multiracial kids. It's a beautiful, wonderful mix in our school."