FWIW, now all forms of Attention Deficit Disorder are referred to as ADHD or Attention Deficit Hyperactivity Disorder; even the non-hyperactive subtype. Not intuitive but I don't make the terminology.
Anyway, everyone has some ADHD traits. While we like to look at ADHD as a binary yes/no diagnosis the truth is that we all fall somewhere on a continuum. At a certain point the symptoms become pervasive enough to make it difficult to be a productive student or employee; it is at this point where it is considered a disorder.
By far the largest contributor to ADHD symptoms is heredity. In utero alcohol and heavy metal exposure appears to be another strong contributor. Some people claim that vaccines and artificial food dyes contribute but there is no objective data supporting this theory.
Many people with ADHD learn to manage it on their own. After years of struggling they learn tricks to stay organized and to overcome procrastination and their tendency not to finish tasks. These people are not cured but simply have learned to deal with it much the way someone who is physically disabled might learn to manage with it.
ADHD is something one has or does not have for life; I think of it more as a personality type than a disorder. People with ADHD need more stimulation than most people which makes them easily distracted from and bored with mundane tasks. The drive for stimulation also allows them to focus like a laser on things they find interesting. The paradoxical ability of people with ADHD to hyper-focus on complex tasks can be an advantage in certain areas. It has been estimated that 85% of all computer professionals have ADHD. Their ability to hyper-focus gives them an advantage solving hard technical problems compared to their non-ADHD peers.
So the sort answer is that you are born with ADHD, you don't develop it though your environment may determine how well you learn to manage it.
You are born with it, but you may not notice it until your older. I'm 15 and I just got diagnosed last spring, but I know of people who have been in their 20's when they were diagnosed.
you are born with it but it may not show till you are older like im 13 and have just recently been tested for ADHD but still have to get tested again this friday