I was diagnosed when I was 22. It's safe to say that I was always autistic, but depending on your upbringing, it may get missed. In my case, my parents just thought I was quiet and really lazy when it came to schoolwork, but it never changed and I was always on the fringes. After I got put on academic probation in college for failing classes due to barely leaving my dorm, my parents finally wondered if something was up. It helped that my younger brother had been long diagnosed by that time. The connections were finally made. Being an oldest child and raised just before mental health and bullying started being taken seriously in society kind of made my childhood a wasted experiment.
Autism is something you are born with that molds who you are. I am fortunate that I was born on the high-functioning part of the spectrum.
As for how it relates to crossdressing, I know there are higher rates of LGBT within the population. The best explanation I have seen is that autistic people lack the social inhibitions that neurotypical people may have. They are who they are regardless of social stigma. It's good and bad because the negative emotions from people not accepting or understanding who you are can be extreme. Personally, I'd say I sometimes feel trapped inside my own head because I can't articulate what I feel like a "normal" person would, especially in real life.
I first came across crossdressing and also female masking when I was 11 or 12 and it's now been 20 years and I've never tried to shake it. I have anxiety over how other people would handle it if it ever leaves the closet, but I know it's just part of who I am.