We started suspecting Ben might have ADHD when he was about 4, we were referred for testing when he was 5, and he was officially diagnosed at 6, right after kindergarten. Technically he's diagnosed with combined type because he struggles with both focus and impulsiveness, but he's not hyperactive. The first red flag for us was his extreme distractability. At 4 he couldn't make it through an age appropriate task (like putting on his pants) without getting distracted and needing to be redirected multiple times. Cleaning up toys just didn't happen. If he had a dumped bin of cars in front of him, we had to coach him through it one car at a time (pick up the red one, pick up the blue one, etc) and he still had trouble seeing it through. He focused beautifully on things he was very interested in, but it was nearly impossible to get him to stay focused on things he didn't want to be doing. His impulsivity came through in lots of ways, but one of the most obvious was that he frequently did things he knew he shouldn't, like hit or splash sink water ALL over the bathroom. Lauren was 3.5 when Ben was diagnosed, and I could trust her to follow rules and make good choices much more than I could her 6 year old brother.
Ben has other issues as well: high functioning autism, a sleep disorder, sensory processing disorder, and a motor planning disorder. For most of his diagnoses there's no easy way to help; it takes hard work, therapy, and time. So when he was diagnosed with ADHD it was an easy choice for us to try meds; if there was anything we could do to make things easier for him, we felt like we had to try. We found one that worked well for him right away, and it helped a lot.
Ben is in 3rd grade now and doing so much better. At home we don't see a ton of difference in medicated vs. unmedicated behavoir so he's med-free on weekends. (He used to need meds at home, too, but he has matured a lot. We still see the effects of the ADHD, but it's much less disruptive than it used to be.) Meds still make a big difference at school. I was waiting for a new prescription so he hasn't been taking his meds for a week or so. His teacher didn't know that, and I got an email from her yesterday saying that he'd been needing a lot more support than usual to stay focused and on task.
Good luck with everything!