Certainly there are adults with autism or Aspergers. These are developmental disorders which are not outgrown although mild symptoms may never be diagnosed.
YES. They don't grow out of it.
My father in law is 70 & went for most of his life undiagnosed as Asperger's. It was not until we moved in to care for my mother in law that we made the link ourselves. Two of his grandsons were diagnosed, 1 with autism & 1 with asperger's but there aer family members still quite erluctant to believe it could be possible. Prior to this we assumed he had some kind of depressive disorder as he had been out of work, and out of the community in many ways for a long time. Also, because he was a mathematician, his behaviours were just considered 'eccentric' by his children & wider family but never really questioned - he has very set routines about reading set numbers of pages in multiple books at a time, and has read some books hundreds of times, etc.
I think though that it could be quite difficult for adults being diagnosed later in life because there can be a lifetime of pain & confusion prior to the diagnosis, which i do at times see in my F-in-Law. At this stage i think there is a great deal of interest in assisting asperger's children to grow & become adults able to live with success in the wider community (now that there is some understanding out there) but maybe its less so for the aging community who have already maybe been through a lot in their lifetime.