How old should a child be before it's reliable to diagnose him with adhd?

Depends, about age 6: In most cases, age 6 is generally the youngest most children are diagnosed with adhd. As there are such variations in normal childhood behavior, under this age, normal active children may be mislabeled with the condition. However, it is possible to even diagnose a 4 year old if the clinical situation is completely clear and the child has had an evaluation. Best to discuss with your doctor/.

As soon as ....: If the presenting symptoms of inattention, impulsivity and /or hyperactiviy are causing significant impairment, have been present in more than one setting , and for more than 6 months;and if those symptoms are not better expained by another behavioral, developmental or mental illness, that child meets most necessary criteria for the diagnosis.The sooner we diagnose and treat the better !