I was diagnosed with ADHD when I was like 8 or so, which may or may not have been accurate. I was definitely over the top energetic to the point where it was disruptive, but I don't know that that's really a mental disorder. I think they just stuck the label on me so they could deal with it more easily. I was put on Adderall (amphetamines) for it and the medication mellowed me out without any really significant side-effects, so I guess this one could really go either way. I didn't have any problems when I went off it when I was like 15 or so, so I strongly suspect it was just me being a kid.
ADHD is a weird one because, to the best of my knowledge, the symptoms can lessen as you mature or they can not. The problem with ADHD is that people often only consider the "hyperactivity" part and forget the legitimate problems that come from the "attention deficit" part. People diagnose it based almost entirely on "H" when the "AD" is the more significant part. Not every hyperactive kid has ADHD and a lot of people with ADHD display no hyperactivity at all. That's why I feel like the now "outdated" term of ADD was probably better.
From a broader perspective though, while over diagnosis/misdiagnosis is definitely a thing with some conditions I think that the greater awareness of mental health issues is probably more responsible for why it seems more common, although perhaps also ironically responsible for those who are misdiagnosed to an extent.
In fact, while greater societal awareness has been, without question, a very good thing for people who actually suffer mental illnesses, I'd go as far as saying an unfortunate side-effect is that it opens the way for misdiagnosis and also
self-diagnosis. Now, that's not to say that self-diagnosis is always wrong. I self-diagnosed my depression and this was later confirmed by my doctor and psychologist working in tandem. What I'm talking about is the people who don't know the difference between being depressed and having depression... and yeah to an extent the people who cry depression all over social media or whatever because it feels good to have people worry about them. I feel like a lot of people go through a bad phase, realise that the way that makes them feel sounds a lot like depression and then assume that they have the illness and there's a lot of people in general who misconstrue what depression is. That is to say, people who think feeling sad for a long period of time
because of a specific stimulus is the same thing as clinical depression, which is an illness caused by a chemical imbalance in the brain. I don't think I need to explain that last group of people I mentioned, they're in the minority and while they contribute a lot to the stigmas and misconceptions around mental illness, I'd rather we tried to help them the same as anyone else because that's better than mistaking someone with a legitimate problem for someone seeking attention.
So yeah, to expand on my earlier post, the seeming surge in mental illnesses isn't actually a result of them becoming more common. The vast majority of the apparent spread of mental illnesses is simply because we as a society have become more aware of mental illnesses as a health issue and not just people being babies or assholes. This awareness has also lead to a smaller group of people who are misdiagnosed or self-diagnosed who claim to have, or believe they have, mental illnesses they don't have. What's most important though, as I said, is making damn sure proper help is given to every single one of those people because it's very easy for a person with a legitimate problem from the first group to be mistaken for one of the people in the second. Be cautious with things like medication, but make sure everyone has access to what they need to ensure good mental health.
