The youngest kids in class may not be acting up, says one researcher, they just might be acting their age.

"A five-year-old is poorly behaved compared to a six-year-old and that's when these diagnoses are made," Todd Elder said.

Elder is talking about ADHD. The MSU economist and author of a new study, said the youngest kindergartners were 60 percent more likely to be diagnosed with the disorder than the oldest children in the same grade.

The cut-off date to enter kindergarten in Michigan is December first. Elder said those kids are far more likely to be diagnosed with ADHD than those kids born December 2nd, because those kids head to kindergarten the next year.

"The problem is kids are being compared to kids in the same grade to evaluate their behavior and they should be compared to kids who are the same age," Elder said.

"A child's attention span is approximately one minute per year of age," Doctor Ian Fox said.

Dr. Fox -- a board certified pediatrician at Ingham Regional Medical Center -- called Elder's findings "completely valid"

"A child who is 4-years-old at the beginning of school is not as attentive as a 6-year-old," Dr. Fox said.

Fox said it's important to evaluate each child differently. Look to see if he or she has behavior problems in two different settings, not just at school.

"Rule out the other possibilities first," he said. "Plus, not every kid needs to take medication."

Elder said his findings don't point to why this is happening, or what to do next, he simply said it's important parents take it into account.

Elder said children are much more likely to be prescribed stimulants by the 7th or 8th grade, more so than their older counterparts, which wastes an estimated $320 - 500 million a year in unnecessary medication.

Doctor Fox said there are long term studies out there, and none suggest that it hurts your child to give them the medication.

For more information you can go to www.add.org