Depends on symptoms
has two major parts--a) poor focus and attention and b) high activity and impulsivity.
Kids with the latter tend to be diagnosed earlier because they don't fit into social settings such as preschool. Those with both--adhd, combined type--typically are diagnosed at about age 7 (6-8) because by then peers don't have those behaviors. Those with fhe former may not be diagnosed till age 9-10.