Robert Stern, PhD. at Boston University, discusses the recent study that showed that kids who play football before the age of 12 are twice as likely to develop behavioral issues. (4:10)

A study released Tuesday suggests that children exposed to tackle football before age 12 are at greater risk for incurring later-life brain issues than those who started playing after that -- prompting one of the lead authors to say in an interview he now doesn't "think there should be youth tackle football."

"I really wish I could say I was surprised" by the results, Robert Stern, a Boston University neuroscientist who has been studying the connection between repetitive head trauma and later-in-life neurocognitive issues for the past 10 years, told ESPN in an interview. "Instead, it was more, 'Oh yeah, this really is a big deal.' And it's just one more piece of the puzzle that, at least when it comes to youth football, has now gotten me over the edge to say, 'You know, we shouldn't be having our kids hitting their heads over and over and over while their brains are developing this way.'"

The study, published Tuesday in the peer-reviewed journal Translational Psychiatry, suggests that children exposed to tackle football before the age of 12 are two times more likely to develop behavioral and emotional impairments later in life than those who start playing after that age. It also indicates that kids who start playing tackle before 12 are three times more likely to develop symptoms of depression later in life than kids who begin playing at 12 or older.

Stern acknowledged that the research released Tuesday has limitations and that the science has "a long way to go." He noted that the subjects in the study were not random but instead had volunteered to participate. As well, the research was conducted through phone and online surveys, making it less robust than had the examinations been performed in person.

The study is believed to be the first to show an association between early exposure to tackle football and long-term brain issues in a group of athletes who didn't go on to play in the NFL. The research examined 214 former players -- 146 of whom didn't play beyond college -- with an average current age of 51. The researchers said they set 12 as the threshold age "because the brain undergoes a key period of development and maturation between the ages of 10-12 in males."

Stern said he was not prepared to offer a specific point at which kids shouldn't play ("I don't know if there's a magic age," he said), but he did caution against reading the findings to suggest the brain was safe if kids waited until 12 to begin participating in tackle football. In a news release, Stern said that "more research on this topic is needed before any recommendations on policy or rule changes can be made."

"Overall, we found the younger that kids started to play, the worse the risk," he said. "... Having them exposed to so much repetitive head impact during these critical periods, I just don't understand how we can keep doing that."

Robert Stern co-authored a report that suggests children exposed to tackle football before the age of 12 are two times more likely to develop behavioral and emotional impairments later in life than those who start playing after that age. AP Photo/Stephan Savoia

Stern said the results reinforced a concept that researchers at Boston University have been espousing for several years: The issue is not related to single-incident concussions but rather repetitive head trauma and extended exposure.

"This should make people aware that we're not focusing on concussion -- that this is just playing the game," Stern said.

In July, researchers at BU announced that out of 111 brains of former NFL players examined posthumously, 110 had chronic traumatic encephalopathy, the brain disease that has been increasingly associated with football. Some researchers, including several affiliated with the NFL and other sports organizations, have suggested the BU group has oversold its findings, causing unnecessary hysteria. The BU researchers acknowledge their data set is skewed, making it difficult to assess the true prevalence of the disease.

The new study comes one week after the Canadian Football League announced it was effectively eliminating contact from all regular-season practices. Two days later, the CFL and NFL jointly announced a youth flag-football initiative in Canada.

The research builds off previous work conducted by Stern and colleagues that looked solely at former NFL players. Published in 2015, that study concluded that those who started playing tackle football before 12 showed worse neurocognitive results than ex-players who began at 12 or older.

Three months after that study was released, several NFL-affiliated researchers, in a paper published in the American Journal of Sports Medicine, said they were unable to replicate the findings of Stern's group. The authors concluded that after examining 45 retired NFL players, they "found no statistically significant association" between starting to play tackle football before high school and developing neurocognitive issues later in life. Three of the co-authors -- Drs. Ira Casson and David Viano and neuropsychiatrist Mark Lovell -- were once members of the league's Mild Traumatic Brain Injury Committee, which was disbanded in 2009; a fourth, Gary Solomon, is a consultant to the Tennessee Titans; and the final author on the paper, Dr. Allen Sills, was named the NFL's chief medical officer earlier this year.

Stern, asked about the NFL-funded study and its findings, said: "We designed this study and analyzed the data to assure that any previous critique of this line of work would be clearly addressed."