According to Pubmed-Health

The ["ADHD"] diagnosis is based on very specific symptoms, which must be present in more than one setting.

Children should have at least 6 attention symptoms or 6 hyperactivity/impulsivity symptoms, with some symptoms present before age 7.

The symptoms must be present for at least 6 months, seen in two or more settings, and not caused by another problem.

The symptoms must be severe enough to cause significant difficulties in many settings, including home, school, and in relationships with peers.

Source: ADHD