Welcome to the boards.
I was diagnosed around the same age in college. One of my psychology professors noticed I prefered to tuck in behind a chair in a window recess and read, instead actually using the chair. I could see out but people couldn't see me from the hallway. I always had my headphones in, never really interacred with those I didn't know and had used a yoga ball for a desk chair since early elementary school. My professor asked me if I had been screened. I said no, but I had been toting an ADHD diagnosis since kindergarten.
At that point I had not heard the term in a context I really understood. I had a search term and the college library. I dug in. The Rain Man sterotype was still the prevailing idea of what autism was. Growing up in small town middle America with a single parent, I already had a label, but the more I read the more things made sense. Then I talked to my mom and my long time psychiatrist.
Both of them told me I had met all the criteria for an autism diagnosis clear back in preschool, the ADHD factors were still there, but at the time you could not be ADHD and autistic, ask the DSM-IV about that marvelous bit of stupidity. It was finally revised in the DSM-V along with the term introvert being removed as a symptom of mental illness in 2012.
I was given the ADHD diagnosis in order to have a little protection from the social stigma of autism. There were only two other kids in the school system at the time that I knew who were diagnosed autistic. A brother and sister pair, about a year apart in age, but both were very clearly on the spectrum. In today's terms it was ASD Level 2. Both liked to walk the school halls reciting favourite movies scene by scene, with sound effects. They never engaged with others, never bothered anyone else, they were just always part of the school group.
As my ASD was a level 1, I had a much easier time blending in. I learned how to mask and well, but college was still new. There was a lot of overwhelm, so I sat behind a chair to decompress. It became a habit and a safe zone. After talking with Mom and my doctor things made a lot more sense.
I wasn't happy about being kept in the dark, but I understand why they did what they did, even though my autism is by far and away the dominant divergence. They updated my records the same day. And while I still had to come to terms with it, things now made a lot more sense. Akin to getting a new pair of glasses and everything comes into sharp focus.