The evidence is not in yet about what causes ADHD. We do know, however, that not all children with ADHD inherit it from their parents. You might want to have a look at Mary Fowler's wonderful article about ADHD and its causes at FamilyEducation.com.
Do kids grow out of ADHD? We used to think so. Now, however, we know that anywhere from 60-80% of kids still exhibit symptoms after puberty. Sometimes, the symptoms may change, as kids get older. For example, someone who was hyperactive as a kid may lose that overt behavior, but still show signs of impulsiveness and inattention as a teenager or an adult.
For more information, you might also want to go to the Children and Adults with Attention Deficit Disorders (CHADD) website at http://www.chadd.org. You can also find out about local support groups for parents who have children with ADHD at that site.