About 6.4 million children in the United States have been given diagnoses of attention deficit hyperactivity disorder (A.D.H.D.). But the condition — characterized by impulsiveness and difficulty sitting still and paying attention — is also being recognized more in adults. The challenges faced by those with A.D.H.D. are daunting and deeply personal. Here, in their own words, are the stories of adults and children coping with A.D.H.D.