Wikipedia tends to be my "go-to" place for trying to find unbiased info, and it says that the proportion of children meeting the criteria drops by half in the 3 years following diagnosis. So I would guess they either never had it in the first place, or they grew out of it (brain maturity or whatever).
"An 8 year follow up of children diagnosed with ADHD (combined type) found that they often have significant difficulties in adolescence, regardless of treatment or lack thereof.[120] In the US, less than 5 percent of individuals with ADHD get a college degree,[121] compared to 28 percent of the general population aged 25 years and older.[122] The proportion of children meeting criteria for ADHD drops by about half in the three years following the diagnosis and this occurs regardless of treatments used.[123][124] ADHD persists into adulthood in about 30 to 50 percent of cases."
