I was finally diagnosed with ADHD at the age of 60. It was a massive relief to realise that I wasn't stupid, or rude, or lazy, or all the things I'd spent most of my life feeling I was. I went for diagnosis because my brother had just been diagnosed & the first thing he did was call me up & tell me to get it done myself.
Everyone with ADHD is slightly different from each other, and actually he and I can be complete opposites in how it manifests (he can't concentrate in the office, I can't at home for example), but one of the things we do have in common is the drive to research everything to death. Then talk it to death!
One of the things we've learned is that you don't "develop" ADHD OR "grow out of it", you either have it or not as it's a brain function and chemistry thing you're born with. People don't suddenly develop ADHD symptoms, they are there all the time though of course changes in life like extreme stress, breakdowns etc can mean they become more prevalent.
Another thing we've leaned is that it has no bearing on whether you're a nice or nasty person! A nice person with ADHD will often be wracked with guilt at the very thought they may have upset someone, whereas some who use ADHD as an excuse for being bad is just doing that - using it as an excuse.
One way my ADHD manifests is that I over talk, talk too loudly, get excitable in conversations etc. I may blurt out something without thinking, but that is never anything nasty, or with unpleasant intent, or cruel; I just have no or very little filter. This is why I have made it very clear to everyone at where I work that I have ADHD and this is one of the ways it manifests, so they realise this and have had it made clear that I'll never take offence if they ask me to calm down, or stop talking when they are busy, or be a little quieter. But they also understand that it means that in certain aspects of work, e.g. a need for hyper focus on critical tasks, or the ability to jump between 3 subjects at once, or see unobvious links between things, it makes me a major asset to the team.
So typical of my ADHD, I've burbled on for far too long & got away from the point I was going to make and completely left out!
Which is that many of the traits shown by people with ADHD are things that almost everyone does now and again - but with ADHD this is the entirety of our lives. Everyone occasionally forgets their keys or something, with ADHD if you don't always put them in exactly the same place using the same routine then you lose them every other time. Almost everyone is occasionally late for something, with ADHD it's either almost a compulsion to be late to everything (my undiagnosed DSis), or you go the opposite way & plan everything to the death and add contingency into every step of the way and end up arriving way too early (me). Most people forget something in their weekly shopping now & again, I write list after list then either forget to take it with me or forget it's in my pocket every single time - even when grocery shopping on line!
That's the problem with people depending on the on-line tests - they ask questions about things that are normal behaviours if they are occasional, but the people talking the test assume they happen more often than they do because they are thinking about those specific things while they do the test. So their test results come out probably higher than they may otherwise.