I have Attention Deficit Disorder, and I am 50. I was diagnosed when I was 44. The disorder, which some say is overdiagnosed in the United States, affects about 5% of the population. Adults are often diagnosed when their children are, ironically, because there is a genetic component to the disorder.
I always had the wrong idea of what ADD was. I thought it meant you couldn't pay attention to anything or finish anything, and that certainly wasn't me. I could read voraciously (as long as it interested me). I could work on a project without stopping for hours (as long as it interested me). In fact, I could get SO focused on something (as long as it interested me!) that I would forget to eat, go to the restroom, or watch the clock.
The key part, though, is the "as long as it interested me" part. If you take a child or adult who has ADD and give them routine tasks or chores, they may fiddle around for hours with distractions and lack of focus. This is me, to a T. Give me a piece of artwork to do and I will be focused for hours. Give me a checkbook to balance and I guarantee you it will take me twice the time and twice the misery that it will take a person who doesn't have ADD (even if there is plenty of money in the account).
Learning I had ADD gave me insight into a lot of things that happened as I was going through school and entering the workforce. In the right environment, an adult with ADD may never guess she or he has it, especially if they were able to choose a career that appealed to their strengths. But in the wrong environment -- one with many routine chores, tedious meetings, or rote tasks, for instance -- a person with ADD is simply not going to thrive. I could look back and see where I succeeded and where I failed. My room (or, later, my apartment) was always a mess, but I always did well at work. I would buy presents way ahead of time, but always be late on sending bills. If it was routine, it was a killer.
Anything here sound familiar? You might be interested in reading the questions that diagnosticians use to diagnose ADD in adults, from the National Resource Center on AD/HD:
http://www.help4adhd.org/en/treatment/guides/WWK9
And my favorite book title on the subject? Kate Kelly's "You Mean I'm Not Lazy, Stupid or Crazy?" Because those are names ADD folks get used to calling themselves when the simple things just don't seem to work.