Adults don't receive many services. The laws and supports are changing, though. For now, the best census of adults with ASDs was done in the U.K. by the National Health Service.
The autistic adults do exist, they just were not counted in the past. Again, from 2009:
On Sept. 22, England’s National Health Service (NHS) released the first study of autism in the general adult population. The findings confirm the intuitive assumption: that ASD is just as common in adults as it is in children. Researchers at the University of Leicester, working with the NHS Information Center found that roughly 1 in 100 adults are on the spectrum — the same rate found for children in England, Japan, Canada and, for that matter, New Jersey.Read more: http://www.time.com/time/health/article/0,8599,1927415,00.html#ixzz18WUKidww
From the U.K. report:
A study of rates of autism spectrum disorder among adults suggests that one in every 100 people over the age of 18 has the condition — broadly the same as that cited for children.
The data, collected by the NHS Information Centre, is the first to show how autism affects people over the course of a lifetime, concluding that it is similar across all ages.
People in more than 4,000 households in England were asked a series of questions aimed at assessing their psychiatric health. The results were used to identify adults with an autism spectrum disorder, including Asperger’s syndrome.
The study was funded by the Department of Health. It found that rates of autism were higher among men (1.8 per cent or 1:52) than among women (0.2 per cent). This reflects studies in children, which have shown higher rates among boys (1:48) than girls.
The report also found higher rates of autism among single people, among men with no university degree and among men who rent their homes rather than those in other types of housing.
Many of these men were previously undiagnosed.The last line, most adults went undiagnosed, should not surprise us. Diagnostics changed dramatically within the last 20 years.