I was diagnosed with ADHD at 19 at university. I'm now 42 and struggle on a daily basis. Example: I've got it in my head that I have a committee meeting tonight at 6.30, no, it is Thursday. It's in my calendar as Thursday so just why???!!! I'm an academic and I am forever masking.
Looking back at being a kid a lot was explained by my diagnosis - my apparent cheekiness, bad behaviour, constantly being told to 'behave yourself', constant curiosity, my dad going crazy 'she's not going to be a guinea-pig' when an assessment at school was suggested, hyperfocus on things that interested me, obsession with science, being made fun of by family members.
I'm glad I wasn't diagnosed as a child as my dad would have said it was a load of old rubbish and I was using as an excuse to get away with bad behaviour. It was explained when I was in tears in my first year at university and struggling with a particular module. I got a diagnosis, I got help and got a conceded pass in that module (thank goodness it was a first year one!)
I think that being undiagnosed as a child and adolescent led to me internalising the fact I was a 'bad girl' who 'couldn't behave' and this led me into some upsetting situations with friends, boyfriends and jobs. I was sacked from four admin jobs because I couldn't follow instructions or understand what was expected of me. I was mortified and took from those experiences that I was useless and unemployable. Two long-term partners didn't 'get' me and it led to lots of arguments, accusations of laziness and stupidity. I didn't even tell them about my diagnosis. Well, I told the second, eventually, and he said I was using it as an excuse. He said he felt like my carer. He was always argumentative and exasperated with me. They are both exes.
I think if I had been diagnosed as a child I'd have had an easier ride as a teenager and avoided a lot of silly choices. Now I am so busy, things fall off the edge, though, and I wonder if I will just get worse!