More and more adults are now being diagnosed with ADHD. Part of the reason is that ADHD is now recognized more and more by physicians and the public awareness of the disorder. I often see children in my practice with ADHD and notice that the parents have very similar symptoms of impulsivity and distractibility. Sometimes the parents even admit that they were similar to their child when they were younger. About half of the children with ADHD outgrow many of their symptoms when they reach adulthood. Many move into careers and jobs that fit their high energy and high activity personalities. One of the things adults and children can do to cope with jobs and schoolwork is to break the projects into smaller manageable pieces. There are excellent books and programs available to help patients focus and stay on task such as Total Focus and Driven to Distraction.
There is no age-limit on a person to develop ADHD symptoms. You are with it when you have attention deficits leading to easy distractibility, inability to concentrate on any ongoing conversations, inability to filter out outside/extraneous stimuli, difficulty in maintaining attention on reading or tasks causing repeated forgetfulness, often misplacing or losing things, forgetting plans, appointments, purse, car keys etc.