More cases of ADHD are being diagnosed at a younger age, with a full third of U.S. children displaying attention deficit hyperactivity disorder being diagnosed before the age of six, a federal report indicates.

This is in spite of the existence of very few valid tests that can support such diagnoses in children so young, the report authors point out.

There is some debate among experts about whether the study suggests an overdiagnosis of the condition, whose symptoms include inattention, hyperactivity and impulsive behavior, all of which can impact a child's ability to learn.

"Although guidelines and instrumentation for diagnosing preschool children, for example, are weaker, the condition itself is developmental and expected to exist in preschool," says Joel Nigg, director of the division of psychology at Oregon Health & Science University in Portland.

"So, many of those young diagnoses may be valid," says Nigg, who was not involved in the study reported by the U.S. Centers for Disease Control and Prevention.

Researchers at the U.S. National Center on Birth Defects and Developmental Disabilities interviewed nearly 3,000 parents of children diagnosed with ADHD.

Around half the children had been diagnosed before age seven, and more than a third had been diagnosed before age six, the researchers found.

In three out of four cases in the second group, it was a parent or other family member who first became concerned about the child's behavior and identified signs of ADHD, they noted.

"We did see that in the vast majority of cases a family member of some kind was the first to express concern for behavior or performance," says study lead author Susanna Visser.

Guidelines released in 2013 by the American Academy of Pediatrics may have helped raise awareness and spur those new diagnoses, she suggests.

Schools have also been instrumental in noting initial symptoms, the report says; around one-third of children with ADHD first had their symptoms flagged by someone at a school or day care facility.

There are benefits of early diagnosis, Visser says.

"In general, once the symptoms start to cause impairment, the child and family can benefit from treatment," say Visser, an epidemiologist. "For kids under 6, behavior therapy can benefit."

Cases of diagnosed ADHD have risen rapidly in recent years, the report authors say, up 42 percent from 2003-2004 to 2011-2012.