HACKENSACK, N.J. — Melissa Sietsma suddenly fell silent, her voice trailing off and hanging softly over the phone line.

The Fair Lawn, N.J., native had been recounting the violent kicks, elbows and falls she had suffered as a teen in club soccer, resulting in an estimated 15 concussions.

And then she froze, unable to remember what came next in her story. The shadows of her mind had stolen another memory.

Sietsma is just 22 years old.

The former college player can no longer ignore the little signs, the steady progression of memory lapses and concentration difficulties caused by her concussions. And they are not going to go away.

"You sort of wonder how bad is this going to be when I'm 85," said Sietsma, a student at The Evergreen State College in Washington State. "What sort of ramifications when I'm 85? Will I be senile at 35?"

Retired boxers and NFL players have come to embody the debilitating effects of concussions. But new research suggests your son and daughter are also at risk — both today and in the decades to come.

Children and teens who suffer concussions can be affected for months with post-concussion syndrome, which can cause symptoms such as depression, headaches, insomnia, mental fogginess and even IQ-point loss.

Now there is evidence those repeated "dings," "bell rungs" and concussions children and teens sustain in sports can set them on the path to the kind of cognitive deterioration seen in generations of pro football players.

Researchers last year autopsied the brain of an 18-year-old high school football player who had died and detected chronic traumatic encephalopathy, or CTE, a debilitating, degenerative brain disease best known as an affliction of retired NFL players, most of them older than 50.

This month, scientists said a brain autopsy of a 21-year-old University of Pennsylvania football player who killed himself in April showed evidence of the same disease. The only known cause of CTE is repeated trauma to the brain, which was found in Chris Henry, a 26-year-old wide receiver with the Cincinnati Bengals who died in a 2009 domestic dispute in which he either fell or jumped from a moving pickup truck. The disease can cause depression, aggression, paranoia and dementia.

These cases among young athletes call into question just how early brain trauma from repeated concussions can begin to take a toll.

Even more alarming, doctors have found evidence that could indicate routine hits in football and other sports that aren't jarring enough to inflict concussions can still cumulatively result in the same condition. The Pennsylvania player, Owen Thomas, was never diagnosed with a concussion or even complained of a headache, according to his parents.

Dr. Thomas Bottiglieri, a concussion expert who practices in Englewood, N.J., said there's no question that young athletes like Sietsma who suffer repeated concussions are in danger of developing cognitive problems.

"We just don't know when it happens, when it starts," Bottiglieri said. "A young person, 15 concussions, maybe more, maybe consistently playing with a head injury, she's going to have some issues."

Still, Bottiglieri cautions that young athletes and their parents should not be frightened after a concussion. Given time to heal, there is no danger in returning to play.

The "major concern" is repetitive concussions, especially among athletes who return to play before they have fully recovered, he said.

Concerns about the effect of repeated hits on still-developing brains have surfaced at a time when doctors and athletic trainers are witnessing a rise in reported concussions among young athletes, which they attribute to higher awareness of the injury.

The U.S. Centers for Disease Control estimates Americans suffer between 1.6 to 3.8 million sports and recreation-related concussions each year. High school athletes in contact sports — football, hockey, wrestling and soccer, to name a few — have as much as a 19 percent chance of suffering one each season, adding up to about 400,000 each year. Cheerleaders also are especially at risk.

A study in this month's issue of the journal Pediatrics indicates the number of emergency room visits for concussions suffered in organized team sports doubled among 8- to 13-year-olds between 1997 and 2007. Among 14- to 19-year-olds, they more than tripled.

Young athletes are more susceptible to concussions than adults and take longer to recover. Girls are more at risk, possibly because they tend to have weaker necks that cannot take as much force.

Once an athlete is plagued by post-concussion syndrome — 5 to 20 percent are — the recovery process can be difficult and prolonged.

There is just one universal treatment: Rest. Dr. Joseph Rempson, co-medical director of the Concussion Center at Overlook Hospital in Summit, N.J., does not even want his patients in school as long as they have symptoms.

"They're not learning anything, and they're failing" classes because they have difficulty thinking, he said. "They need cognitive rest, not just physical rest."

Most athletes need two weeks to overcome their symptoms, and 40 percent need even more time, according to Micky Collins, assistant director of the Sports Medicine Concussion Program at the University of Pittsburgh Medical Center.

Collins said there are some who are lost causes before they even reach 20 years old, victims of repetitive concussions that weren't treated.

"There's some patients who can't get better," he said. "Mood changes, depression. In 99 percent of those cases, the kid was poorly managed and they kept playing.

"They get hit again and again and again, and that's a road you can't come back from."

There is no simple answer to the question, how many concussions are too many?

Severe symptoms stemming from rather light blows and lingering side effects are telltale indicators athletes should consider no longer playing, according to Rempson and Collins.

"When we start to see routine things cause concussions or a youngster having a prolonged recovery, we need to stop and think," Rempson said. "You can be done with one concussion."