A new medical study has found that children who play football before age 12 suffer mood and behavior problems later in life at rates significantly higher than those who take up the sport later.

The study, published Tuesday in the medical journal Translational Psychiatry, showed those who participated in football before age 12 were twice as likely to have problems with behavior regulation, apathy and executive functioning — including initiating activities, problem solving, planning and organizing — when they get older. The younger football players were three times more likely as those who took up the sport after age 12 to experience symptoms of depression.

"Between the ages of 10 and 12, there is this period of incredible development of the brain," said Robert Stern, director of clinical research at Boston University's Chronic Traumatic Encephalopathy (CTE) Center, who co-authored the study. "Perhaps that is a window of vulnerability. . . . It makes sense that children whose brains are rapidly developing should not be hitting their heads over and over again."

[Poll: Americans mixed on whether tackle football is safe for children]

Stern said the results were not tied to the total number of years the subjects participated in football or the highest level at which they competed. Perhaps most important, the findings also were not affected by the number of concussions the former players reported, he said, meaning the dangers posed by football can't be boiled down simply to big hits to the head. Researchers are increasingly focusing on the effects associated with the accumulation of smaller hits that a player might more easily shake off during a game or practice.

"Concussions are a big deal when it comes to short-term problems, and it has to be dealt with," Stern said. "But the dialogue out there needs to now start focusing on these repetitive hits that are part of the game and their potential for long-term problems."

The Boston University researchers studied 214 former players. Their average age was 51, which means the typical study participant played youth football four decades ago.

Football's impact on young players has been a growing area of research in recent years. While the Boston University study focuses on the possible long-term impact, other researchers have examined the short-term consequences the sport can have on developing brains.

In one recent study, researchers from the Wake Forest School of Medicine followed a group of 25 players, ages 8 to 13, for a single season, measuring the frequency and severity of helmet impacts. The players underwent magnetic resonance imaging (MRI) tests before and after the season, and they showed significant changes in the brain's white matter. None of the participants in that study showed signs or symptoms of concussions, and the players who suffered more hits saw more significant changes to the brain.

As the science evolves and awareness grows, youth football participation has been in flux in recent years. According to the Sports & Fitness Industry Association, participation in tackle football among 6- to 12-year-old boys has dropped nearly 20 percent since 2009.

According to the organization, participation in tackle football for ages 6-12 last year was at 982,000, slightly up from the previous year but down nearly 75,000 from 2012. Participation for flag football in the same age group was at 893,000 in 2016, up more than 200,000 from just two years earlier.

In response to health and safety concerns, USA Football is piloting a new version of the game this year that it calls "rookie tackle," aimed at minimizing the physical toll the sport takes on young players and bridging the gap to the traditional game.

Augmented rules limit teams to seven players on the field, eliminate special teams play, shorten the field to 40 yards and call for players to start from a crouched position, rather than a traditional three-point stance. The organization, heavily funded by the NFL, is piloting the program this year with about 1,000 children in 10 different organizations, and it could introduce it to more cities and leagues in the future.

Stern said childhood involvement in sports is "critical" and has many benefits, but he said there is not necessarily a safe age to introduce children to tackle football.

"Is there a magic age? No," he said. "Our results suggest that age 12 is a meaningful cutoff, but some people might be slower to develop, so for them age 13 or 14 might be the time where their brain is more developed and less prone to problems."