The number of children diagnosed with attention deficit hyperactivity disorder has risen steadily in the past 20 years. The rates of ADHD diagnosis increased an average of 3 percent every year from 1997 to 2006, and about 5 percent every year from 2003 to 2011. This rise in diagnoses has prompted concerns of misdiagnosis and over-diagnosis, and a new report from the Centers for Disease Control may have just added another reason to be concerned.

According to the CDC, 9.5 percent of the children in the U.S. between the ages of 3 and 17 have received a diagnosis of ADHD, about 5.9 million kids. A new study revealed that one-third of these children were diagnosed before the age of 6.

Researchers talked to nearly 3,000 parents of children who had been diagnosed with the disorder, and learned that the average age of diagnoses was 7 years old, meaning many children were diagnosed earlier than that.

Experts cite multiple factors for these statistics.

First of all, the overall increased rate is likely tied to children receiving a diagnosis at a younger age, according to Michael F. Troy, the medical director of behavioral health services at Children’s Hospitals and Clinics of Minnesota.

“In general there’s more awareness of the diagnosis, and once something’s accepted as a relatively common diagnosis, it becomes less stigmatized,” he told CBS News.

Troy also cites a child’s environment outside the home as a factor in ADHD diagnosis — younger children’s symptoms may be easier to identify and subsequently cause problems.

“The threshold to be dismissed from daycare of preschool is much lower than from elementary school or junior high or high school,” he said. “So if you’re a parent, and you have a child with highly disregulated behavior for any reason, it’s easier for somebody to say that they can’t come back to this daycare anymore. And now the parent has a problem that needs to be solved, whereas that wouldn’t happen as quickly for a school-aged kid.”

The difficulty in diagnosing ADHD also lies in the identification of behavior as the disorder, without considering other factors, said Troy. Oftentimes ADHD is a descriptor of behavior but does not include the etiology behind it.

“You could have a severe cough and it could be because of a lot of reasons,” Troy said. “You could have disregulated behavior and have difficulty focusing and concentrating, and it could be ADHD or it could be a combination of other conditions.”

Experts expressed concern that overdiagnosing ADHD can lead to more problems in the future, and they say that this potential for inaccurate diagnoses is the reason parents should closely monitor their child’s behavior.

“Over-diagnosis presents a number of problems, including being improperly labled as ADHD if, in fact, another behavioral or psychiatric problem is the cause of the symptom,” Dr. Danielle Fisher, vice chair of pediatrics at Providence Saint John’s Health Center in Santa Monica, Calif., told HealthDay. “This could also lead to overmedication of such children.”

Additionally, the research found that for about one in five children, it was only family members who provided information to the doctor during ADHD assessment. The American Academy of Pediatrics suggests information be collected from multiple sources, like teachers, coaches, and other adults involved with the child’s care.

"It's important whenever possible to get data from a number of informants," Troy said. "It's the best standard of practice to see how other people judge the behavior of the child and how they are doing in different contexts over and against expected developmental norms."

The study also found that slightly over half of the children with ADHD were diagnosed by a primary care physician, which Troy says highlights how necessary it is for primary care doctors to have access to mental health resources.

"One thing to take away from this is the importance of integrating and supporting primary care providers with appropriate mental health services," he said. "We all know primary care visits tend to be pretty brief. It's very challenging for primary care providers to do the kind of data collection we discussed. It's something we know in the field and something we're working on, but we need to do a much better job overall."