"Evaluation of the Persistence, Remission, and Emergence of Attention-Deficit/Hyperactivity Disorder in Young Adulthood"

"we identified heterogeneity in the young adult ADHD population, such that this group consisted of a minority of individuals for whom ADHD persisted from childhood and a larger proportion who did not meet criteria for the disorder until young adulthood. Our results suggest that adult ADHD is more complex than a straightforward continuation of the childhood disorder...

Although an ADHD diagnosis is more common in boys than girls in childhood, epidemiologic surveys21 of adult ADHD identify a sex ratio closer to 1:1. The larger proportion of women in the E-Risk adult ADHD group is due to a higher number of women with late-onset ADHD joining the population in adulthood rather than childhood symptoms being especially persistent in women. In girls, ADHD symptoms may be less likely to come to the attention of parents and teachers owing to lower rates of externalizing-type behaviors,22 resulting in fewer girls with ADHD diagnosed in childhood...

The late-onset ADHD group showed several characteristics that differ from childhood-onset ADHD, including a dissimilar sex composition (the late-onset group included more women) and lower heritability. Indeed, these differences are consistent with the extant research21,26 on the characteristics of childhood and adult ADHD populations. We found that the risk of developing late-onset ADHD was similar regardless of whether the participant’s twin had childhood ADHD. The extent to which the etiology differs between childhood-onset and late-onset ADHD has broad implications for our understanding of the adult ADHD population."

http://archpsyc.jamanetwork.com/article.aspx?articleid=2522750#Discussion

There isn't a lot of explanation here, but I'm excited to see