I was diagnosed at 41.
I have been informed that I exhibited signs of sensory defensiveness from a very early (prior to one year) age, that a teacher thought I had a learning disability at 6, that a family friend told my mother he thought I was autistic at least a decade before AS was in the DSM and only a couple of years after Lorna Wing's paper. I nearly attended a high school that was explicitly not mainstream, but ended up staying in a mainstream high school, which turned out to be a disaster for me, to the point of dropping out. When I described my history, my therapist asked me how on Earth I wasn't diagnosed as a child.
I don't think this is unique - I think it's common to be diagnosed later.