First: the diagnostic criteria for all autism spectrum disorders states that symptoms must appear before a person is three years old. This means that if a child develops normally until he's six or seven, and then develops autism-like symptoms, he will not receive an autism spectrum diagnosis. Of course, quite a few children develop autistic symptoms as toddlers but are not diagnosed until they're a bit older.
Second: the diagnostic criteria for autism have changed radically, and those changes occurred in the early 1990s. Prior to those changes, there was no "autism spectrum." Around the same time, new diagnoses, including Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS) and Asperger Syndrome, were created. As a result of these changes, people with a much broader range of symptoms were diagnosed as "autistic" than ever before. Most of those people were children at the time of diagnosis.
Third: a rise in autism awareness, combined with a possible increase in absolute numbers of children with autism, has resulted in a huge increase in autism diagnoses among young children.
Fourth: many adults who would, today, be given "autism spectrum" diagnoses are either fully functional (and need no diagnosis) or received other diagnoses as children. Some of those diagnoses may include mental retardation, childhood schizophrenia, or emotional disturbance.
Fifth: because so many children are diagnosed with autism spectrum disorders, there are quite a few professionals with knowledge of and experience with childhood autism. There are very few experts in adult autism. As a result, it is very difficult to find a qualified expert who can reliably diagnose autism in adults.