Doctors have long classified attention-deficit hyperactivity disorder as a childhood condition—one that, in many cases, continues into adulthood.

The Diagnostic and Statistical Manual of Mental Disorders defines it as “a persistent pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development in two or more settings.” To be diagnosed with ADHD, individuals must exhibit multiple symptoms before the age of 12, something most present before age 7.

A new study out of King’s College London may uproot that diagnostic protocol, suggesting the disorder can—in some individuals—develop far later than age 7. The findings have huge implications for the treatment of ADHD, and the way those with it are identified.

The current criteria for ADHD has mostly limited diagnoses to children struggling to sit still, pay attention, or study in classrooms. By 2011, U.S. doctors had diagnosed more than 6.4 million children with ADHD—more than 11 percent of 4- to 17-year-olds. It’s a number that has soared throughout the 2000s, increasing 43 percent between 2011 and 2003, according to a December study published in the Journal of Clinical Psychiatry.

The disorder, while thoroughly studied, has been evolving—even as recently as last year. Previously, doctors believed ADHD to be much more common in boys. Scientists cast doubt on this theory, with a number of studies showing that girls may actually have the disorder in equal numbers. A December 2015 study reported a 55 percent increase in girls diagnosed with ADHD, leading researchers to conclude that it may manifest differently in girls, and thus be overlooked.

The study released Tuesday by King’s College adds a new group that may have been previously overlooked by doctors: adults.

In a study published in JAMA Psychiatry, researchers looked at 2,200 18-year-old twins in Britain for signs of ADHD in childhood. Symptoms were tracked via parent and teacher reports at the ages of 5, 7, 10, and 12. Some 70 percent of the 167 people in the study who showed symptoms of ADHD did not test positive for it in childhood.

“This group showed significant levels of ADHD symptoms and impairment, as well as poor functioning and high rates of psychiatric comorbidity,” the authors write. “Therefore the absence of a childhood diagnosis of ADHD should not preclude adults with ADHD from receiving clinical attention.”

The scientists used to the data from the 167 people with adult ADHD to analyze the differences in their genes from those who acquired it in childhood. They found that those who reported symptoms of ADHD as an adult and not as a child had a “less heritable” version—meaning that their children would not have a higher risk of getting ADHD.

The study built on earlier research in Brazil, New Zealand, and the U.K. that found a high number of adults who had ADHD that was not present in childhood.

Jessica Agnew-Blais, one of the lead researchers, said the study didn’t explore the potential cause of the late-onset cases, but her team is exploring theories.

“We speculated about the nature of late-onset ADHD: The disorder could have been masked in childhood due to protective factors, such as a supportive family environment,”Agnew-Blais said. “Or it could be entirely explained by other mental-health problems. Alternatively, late-onset ADHD could be a distinct disorder altogether. We think it is important that we continue to investigate the underlying causes of late-onset ADHD.”

The researchers said many questions remain, including why late-onset ADHD arises and how similar it is to that which occurs in childhood. Until then, they hope the research will be used to raise awareness about an underrepresented group in the medical world.

“Although ADHD occurs in approximately 4 percent of adults, relatively few adults receive a diagnosis or treatment for the disorder,” Agnew-Blais said. “It is crucial that we take a developmental approach to understanding ADHD, and that the absence of a childhood diagnosis should not prevent adults with ADHD from receiving clinical attention.”