This is a common misconception. ADHD usually doesn't actually grow out like some other mental issues. And for those saying it isn't real, I suggest you check some sources first. ADHD is real, but we certainly have an issue with it in North America. Psychiatrists are quick to jump on ADD and ADHD as a stand-in for bad behaviour/ poor grades. In fact, ADHD is now a measurable issue with the brain, and extensive testing can prove you either have it or you don't.
ADHD can have adverse effects as well as positive ones. If you're the type of kid who just can't sit still in class however, it's more likely that you have ADD or just too much sugar. Those with ADHD tend to be ABLE to focus, but only on one thing. It's widely believed that many who were very proficient in one field but severely lacking in others, near savantism, suffered from ADHD. Steve Jobs for example, was amazingly focussed on his newest products, but couldn't be bothered with his test results from a recent surgery.
For what it's worth, I was diagnosed with ADHD and Dysgraphia (a condition that relates to fine motor movement of your hands, i.e. writing) a long time ago after , but don't medicate it. I can make use of my prescription if I need to, but find attention drugs (adderall and ritalin are the worst by the way) have terrible side effects. I used to visit a centre for ADHD research, run by one of the top Neuro-scientists in North America, twice a week and did some fairly intense brain exercises that help me more now than meds probably could. Eating fairly healthily, going to the gym, and tonnes of willpower help me out in university now, but I never look down on those who ACTUALLY have ADD or ADHD and need to use medication. It really is an issue, not just a "Big Pharma" disease, though I do agree that some are using it as a fallback for other issues.