The attitudes toward/misinformation about developmental conditions on MN is so depressing. Whether intentional or not, the language and implications are awful.
I was diagnosed with ASD and ADHD (and dyslexia, and dyspraxia!) last year at 31. My family were stunned and talked about how they would never have guessed (even though I broached the subject of me being autistic several years ago and was roundly laughed out the room) because the only ND child at my school was a "badly behaved" boy who took ritalin, had a 1:1 and threw chairs at teachers. My auntie said they had all noticed my lifelong "quirks" but thought they were just me being me. My rigid rules about food, my hatred of unexpected noise, the overblown reaction to ever being sung Happy Birthday, the fact I refused to touch certain fabrics or wear certain clothes even as a toddler, my inability to maintain friendships, my weird collections/special interests/obsessive reading/the fact that I spoke like a wise old man as a toddler, the way I have an almost American accent even though I grew up in Edinburgh and sound nothing like anyone else around me.
In reality, my psychiatrist says I'm an absolute textbook example of a late-diagnosed female. My mum basically ran every detail of my life until I was at secondary school, where things became a lot more complex, fragmented and important so it started to unravel. All of my reports said the typical ND girl comments: "Amusedbush is very bright and capable but must focus/stop chatting/ask for help when stuck/do her homework". I've read research papers from shockingly recently concluding that boys and girls with equal support needs in terms of autism were not diagnosed like for like; boys would be diagnosed as autistic based on the presentation of their autism alone but the girls were only diagnosed when they also had a comorbid learning disability.
OP, you mention nobody having any sensory issues or problems with uniform - when I went to secondary school in 2002 and needed to wear a shirt and tie, I was MISERABLE but I couldn't articulate why. I insisted on wearing shirts several sizes too big for me because I can't bear the feeling of tight clothes or anything touching my neck. And yes, I ate my lunch without issue but that's because I had a cheese and ham sandwich every single day for almost a decade. If we were at school together, you would have no idea of my difficulties because it only really presented as me being precocious, bossy and a bit uptight.
We have always existed, it's just that we have been told to "stop doing that; sit still; stop making that silly noise; stop complaining; just eat it; give granny a cuddle; don't be rude; stop shouting; stop being so lazy; stop being overdramatic" our entire lives so we've masked to "pass" as NT. Let me tell you: masking is painful. Physically, emotionally painful and exhausting. And it never actually works - I've stamped down the ND parts of myself my entire life and I've still been bullied and laughed at and called strange/aloof/cold/unlikeable.
Since my diagnosis, my brother, my DH and I have all realised how obviously autistic my dad is. Seriously, PAINFULLY obvious. Then we look back at his dad, who apparently was a "pernickety" and "difficult" man who would have a fit of rage if anyone upset his plans (e.g. if my granny wasn't ready to go out at the agreed time) and every time he found an item of clothing he liked, he bought 5 of it.
Sorry this is so long, I hope it gives some insight.