Can autism first show its signs in adulthood?

No. By definition, some symptoms have to be present by 3 years of age.
No. Autism is a disorder that presents prior to 3 years of age, with impairments in 3 major realms, behavior, communications and social interactions. Perhaps due to increased public awareness and education, the prevalence has been increasing in the past 2 decades. A related condition, asperger's disorder, is also increasingly identified. Unlike those with autism, those with asperger;s do not have delc.
Doubtful but possibl. The symptoms of Autism usually present themselves in young children. However the social awkwardness of Asperger's disorder or of high functioning autism may be mistaken for shyness, not autism. However, it would be unusual for a person to get to adulthood before diagnosis is made.