It's been a few years, but I believe the problem with ADHD kids is that their brain is in the phase little kids have at the end of the day where they get unfocused and hyper just before they fall asleep.
The Ritalin brings their brain up a notch. If you don't need it, it can bring it up too much. But there are adults on Ritalin since the problem is not always outgrown or they don't learn to cope with it. I've know a few adults who should have been on it!
I may have the details wrong, so someone correct me if so.