An ADHD diagnosis (with or without hyperactivity) is based upon a set of observable behavioral characteristics. It is quite common for such characteristics to wax and wane over time, appearing relatively significant at one age and relatively insignificant at another. These characteristics can also be influenced by other factors such as anxiety, depression, fatigue, stress, etc. Some children do appear to "grow out of" ADHD over time and no longer need any type of treatment as adults. On the other hand, the inattentive type of ADHD (which used to be referred to as ADD) often goes undetected (especially for girls) until high school or even later. So it is possible that your son has grown out of his ADHD symptoms while your daughter has grown into hers.