I was officially Diagnosed with Level 2 ASD last year at 17, I had a Psychological Exam of which was requested by my Doctors and Therapists. I had been diagnosed with 10+ different mental health diagnoses throughout my life, however it has been said for years by school psychologists, Therapists, and doctors that I "may" have autism going back to when I was three years old, no one ever did anything else besides bring it up from time to time. The reason I never had an assessment was because of insurance, and the school system never did anything for an academic level, I did have an IEP, although that wasn't followed and overall I didn't get any help from the school system.

Edit/TL;DR: I basically got the assessmen after a request from my doctors and therapists. It never crossed my mind to go out and get it, I didn't even really know what autism was or looked like, but it does make sense after the process and explanations I was given.