"It is an unfair comparison," said Besser.
The study would have provided a more accurate comparison if both groups were enrolled at around the same age and the boys without ADHD were chosen regardless of whether they had problems in school, according to Besser.
Still, Klein's study does add to the question of whether continued ADHD treatment would have helped some of the boys with ADHD into adulthood.
The study comes at the heels of another by researchers in London also released Monday finding that children with ADHD who took stimulants reported feeling better and in control of their behavior.
Stimulants such as Ritalin, Adderall, and Strattera, are considered among many physicians the most effective treatment for ADHD.
"Not everyone needs medication, but when a good assessment is done, these medications are among the safest, not just in psychiatry, but all of medication," said Dr. David Rosenberg, professor of psychiatry at Wayne State University in Detroit, Mich. Rosenberg was not involved in either study.
But prescribing stimulants to children has been a growing controversy among experts and parents. Many who oppose say children who take the medication turn sluggish and even seemingly robotic.
The findings from both studies may underscore the importance of managing ADHD through proper treatment for the individual child, some experts said.
Since each child responds differently to treatment, "the best predictor of a good response to treatment is making an accurate diagnosis," said Rosenberg.
"When it is misdiagnosed, not diagnosed or not treated, the consequences to the child and whole family can be catastrophic," he said.