I have always been both extro- and introverted, with social activity being quite exhausting even when I enjoy it, but also happy to do nothing but sit in front of a PC 24 hours a day. When socially active, I have been pretty great at superficial interactions, I don't even have a lot of trouble making friends, historically - but I am also very prone to misunderstand things others seemed to intuitively understand, and also have a history of changing peer circles and losing contact with people.

I've been young before, but I am old now.

I'm a cishet guy, so I guess I have been a boy.

I have phases of fidgeting and other noticeable stimming, but can also sit completely still, both as a learned/trained thing, stress reaction (to the point of basically dissociating), or when otherwise focused on things.

I both excelled and struggled in school - I excelled at the topics I loved and was able to bullshit a lot thanks to absorbing a lot of general knowledge because my family was all failed, mentally ill and ND academics/intellectuals. But I simply never put in any effort and was outright defiant for anything that did not interest me - and I never did homework, flunked out of classes repeatedly and basically stayed home about 1/3rd of my career as a student, but still managed to finish school - probably because some teachers found that combination of knowledge and defiance "endearing" and they put in a word with the teachers that rightfully found it just to be obnoxious.

"Difficulty regulating" attention hits the nail on the head, and the fact that I thought to myself "But I can pay attention to things, I can even be extremely focused, so it can't be AD(H)D" as a misconception for decades was sadly a very regrettable part of my life that kept me from understanding myself properly.

(I even had a diagnosis when I was a kid, but my parents were rather ambivalent about it, and it was the late 90s/early 2000s, the time when ADHD was presented as "just those damn psychiatrists/psychologists suddenly pathologising normal behaviour, just discipline your kids right" very regularly and with relatively little backlash - probably one of the most damaging pop-culture contrarianisms that affected my childhood/teenage years. I internalised it for decades and just assumed that diagnosis back then must have been a "trendy" mistake, because the most superficial "these are the symptoms" materials I read, never seemed to fit properly - and the people I knew with confirmed ADHD seemed so different from how it manifested for them.)