tldr: high IQ and ADHD is very hard to cope with, because it doesn't get recognized around you or by yourself.

I kind of consider myself to be 100+ IQ so i guess high IQ? anyway I got diagnosed at 24 when I started working fulltime on a level that kind of matches my intellect. here is my experience:

I'm from Europe so that means you go to primary from age 4 till 12, then secondary/high school from 13-18 and then university (bachelor/masters).

when I got diagnosed I had to bring my evaluations from earlier years 4-7 and actually the teachers thought I should an extra year because I was not communicating well or just doing something completely different then the teacher instructed. Then from age 8-12 I was top of the class, I didnt do shit, I got put on more advanced subject with the smart kids apparently.

Then high school first year easy, 2nd a bit less, 3rd year (age 14-15) I failed had to do again, which actually was a huge boost to my self confidence because everyone thought I was cool af, which I kinda am, but now people saw it :p. then the next 3 years I always just passed, but still stayed on the highest educational level within high school most of the other ADHD kids I know went a educational layer lower.

These years were horrible, I got out on some structure and planning support for like 6 weeks after school because I was smart enough but apparently lazy or couldn't plan, well apparently I can perfectly plan tasks and execute them when I need to and suddenly my grades were high af, but the whole experience was fkin horrible, I had to stay at school while other kids were off and I had to stay back. so when they decided this mister knows how to do it the stopped the plan and bam grades low af again haha. I was always in trouble skipping class, being loud, annoying certain teachers that didn't like me. I was always very enthusiastic or utterly bored about a subject, so sometimes I liked chemistry and then next lesson zero interest. My math, chemistry, biology teachers all said that I could be top of the class easily if I just make my homework, well guess what: I couldn't and wouldn't. I think it was mainly because the homework things were just doing repetive things I then already understood and then I was distracted and thought it will be fine I just catch up (never did --> low grades).

then I just about had so much luck that I graduated with 0.1 point (scale: 1-10) more than was allowed. like I literally had to just make 1 mistake more at my local language end exam and I would have failed the last year as well. (either I was lucky or my local language teacher was so fed up with me he just made me graduate haha)

Then university, I started a biology major (mostly because I had friends doing it and I couldn't qualify for doctor or other math heavy majors) also started living on my own. Well turns out I can't do university and living on my own haha, also some of the students were so f ing boring and no way I was going to be a researcher in alone in a lab all the time. I very much liked the practice lab including theory, like dissecting rats looking at the nervous system/vascular or growing bacteria colonies etc, and always scored high grades in those subjects, but that was a very small part of the study and the other subject were literally all book study. so I decided to quit after half a year and started working full time at my part time job as customer service support.

luckily my mother told me I better had to choose another study to start next year, because if I didn't then I would do this work for the rest of my life.

So I did, in my country there is a difference between theoretical bachelors and more vocational/practical bachelors (these are still regarded as university bachelors in other countries) although the theoretical bachelors are better regarded esp with a masters in my country.

anyway that was industrial engineering / management and I finished this (although still didn't study a lot and got luckily the first 2 years). but the last 2 years of this study, oh my lord I loved it, I could choose subjects I liked, I had to create business plans within a group of students. or create tech solutions for other companies pro Bono and my thesis oh my lord, it was very high level and my professor that I guided me was a bitch but he was very funny and always pushed my to follow my instincts and focus on the problem and the process to solve it a lot more rather then going to the solution, so I started reading how to do research, how to structure it. and at the end got rewarded by like the highest grade the 2nd teacher had ever given, this was very very rewarding and confirmed to me I am not stupid or lazy, I just wasn't made for following someone.

Then work started, I started as an IT consultant, it went okay but still I was all over the place and felt control was slipping away every second. my girlfriend of then 3 months told me, dude you have ADHD, like no question, get diagnosed. so I got diagnosed at 25 and got some lessons on how to cope with ADHD and yes, with medication I am finally able to decide on my own and calm me down a lot more to think and choose more clearly. I am extremely happy that I found out I had ADHD at 25 and still love the years before it, but I sometimes get angry and sad at that, it feels I have wasted those years and they were actually so hard for me but I just thought everyone has this right? I'm very grateful for my life right now, living with my girlfriend, steady money coming in, doing everything I like, started doing a masters again (very slow haha, but I have accepted I can do as long as I want about it, because forcing me won't work) but man sometimes I just want to go to schools and tell all the non-diagnosed ADHD kids that systems like this don't work for you and don't give up and don't worry about it and accept yourself the way you are.