Men who want to become fathers might want to think about the implications of their choices, Eisenberg suggests.
"There is data that a man's fertility declines with age," Eisenberg, an assistant professor of urology, wrote in an email. "As such, it may make sense to not wait too long as it may be more difficult to conceive. In addition, there are some potential risks to children."
Eisenberg and his colleagues analyzed 168,867,480 births -- all the live births reported in the US Centers for Disease Control and Prevention's National Vital Statistics System from 1972 to 2015.
The system records births and deaths reported by all 50 states and includes parents' self-reported ages, education levels, races and ethnicities.
"Most data on rising parental ages in the US has been restricted to mothers," Eisenberg wrote, which makes sense because birth certificate data are generally collected from mothers. "We wanted to examine trends in paternal demographics based on the data available on birth certificates since the 1970s."
Over the study period, the portion of newborns' fathers who are 40 or older doubled from 4.1% to 8.9%. Meanwhile, the proportion of dads who were 50 or older rose from half a percent to nearly one in every 100.
The youngest dad for the period studied was just 11 years old, while the oldest was 88.
Asian-American dads -- in particular, men of Japanese and Vietnamese descent -- were the oldest fathers, the study found. Their ages ranged upward of 36 years old, on average. More years of education also correlated with fatherhood happening at an older age during the 44-year study period. Typically, fathers with college degrees are just over 33 years old.
"Take home points" noted by Eisenberg included the fact that paternal age in the United States has risen "across all race/ethnicities, educational attainment levels, and regions of the country." That said, some regional differences existed. Northeastern and Western states showed the highest paternal ages on average, the study found.
"A surprise to me was that more than 10% of birth certificates in the US lack paternal data," Eisenberg said, although the reasons for that aren't clear.
Though mothers tend to be younger than fathers, "the difference between paternal and maternal age has decreased over time," Eisenberg noted. This suggests that both mothers and fathers of newborns are older today, but the average age of mothers is increasing slightly faster than that of fathers.
"These demographic trends reflect our society so if men are delaying fatherhood there are likely many implications such as smaller family sizes," he wrote. "Another possible implication is a higher risk of certain diseases which are more prevalent among older fathers."
One 2012 study
estimated that the male germline -- the genes a father will pass on to his children -- develops two mutations every year; with an advancing average paternal age, inherited mutations in the general population will also rise, noted Eisenberg and his co-authors. Numerous reports have linked older fatherhood with an increased risk of autism, psychiatric illness, neurologic disease such as neurofibromatosis, pediatric cancer and chromosomal abnormalities in children.
"As such, these trends may suggest that we should be seeing more of these occur in children over time," he said.
However, there are positives when older men become fathers, Eisenberg noted. Generally, they are more likely to be stable, with better jobs and more resources, and perhaps most important, they are more likely to live with their children and help with child-rearing.
No need to worry -- yet
Magdalena Janecka, postdoctoral fellow at the Seaver Autism Center at the Icahn School of Medicine at Mount Sinai in New York, referred to the new study as "huge" in an email and said it offers "interesting insights into reproductive trends among different ethnicities and across the states." Janecka, who was not involved in the research, published her own study of older dads this year and found that the sons of older dads had, on average, better educational and career prospects.
Overall, the new study's findings "are in line with the trends observed in other Western countries," Janecka wrote in an email, citing data from the United Kingdom
indicating that average paternal age has risen from 30.6 years in 1991 to 33.2 in 2015.
However, the authors' explanation for this upward trend in the ages of fathers, "including increased use of contraception, increased entry of women into the labor force, and longer life expectancy, may not represent an exhaustive list of possible causes for older parental age," Janecka added.
"Such increase is not just a recent phenomenon," she explained. One historical study
found that the average paternal age in 18th- and 19th-century Sweden was 34.37 years, she wrote. "Similarly, in the UK, women's age at motherhood in 1938
was only a year lower than in 2013," with a dip occurring between those years, she said.
Speculating about underlying causes for the older ages of dads is difficult, she added, given that the new study does not include information about whether these older average ages resulted from delaying fatherhood or extending it.
When it comes to this upward trend in the age of fathers, Janecka does not believe worry is necessary -- at least not yet.
As she sees it, a number of negative outcomes have been reported -- including disorders like achondroplasia, a form of dwarfism, and autism -- but there are possible positive results, including higher IQ for the baby, which may occur when men become fathers at older ages, she wrote. "We still do not know to what extent those associations are due to the effects of age itself and to what extent due to age-independent traits of men who decide to delay fatherhood."
The role of maternal age with respect to these disorders is also unclear.
"One clear message" still emerges from the studies to date, Janecka said: "Contribution of paternal age to those disorders/traits is overall negligible and should not influence individuals' decisions about the timing of parenthood."