Can autism spectrum disorders be acquired or are they inherited traits only, especially in the case of Asperger syndrome or pervasive developmental disorders where they lie on the lower end of the spectrum? If so, does there seem the be a cut-off age, like the onset of adolescence or adulthood? Or can it be acquired at any point in one's life? I understand it can be discovered later in life, but can it actually develop due to one's environmental factors?
Anybody with enough will and persistence can find support for almost any viewpoint they desire concerning autism, depending especially on (lower) quality of source material. However, if you read enough, consistent patterns will emerge in the literature.
This issue of Dialogues in Clinical Neuroscience is all about autism. Although all of it is valuable, I strongly encourage you to work through the wealth of information in Autism risk factors: genes, environment, and gene-environment interactions by Pauline Chaste, MD, PhD. and Marion Leboyer, MD, PhD.
In it you will discover that autism is more and more thought to be a disorder caused by a combination of genetics and environment: genetic factors, epigenesis, maternal environment, drug exposure during pregnancy and a host of other factors, all occurring before of during birth.
There seem to be two types: the slow progressive and the loss of developmental skills type. For this reason the American Academy of Pediatrics recommends screening for autism at 18 and 24 months, though some Pediatricians start screening at 9 or 12 months.
The short answers to your questions:
Autism Spectrum Disorder appears to be genetic and environmental, but that environment is strongly in utero or at birth. So, no, it does not appear that one can acquire ASD as one might acquire, say, chicken pox if unvaccinated.
Although people with autism may take a long time to be diagnosed, it is not 'acquired' later in childhood or adolescence.
Other degenerative neurological disorders can occur later in life, for example lead poisoning or other diseases, but they are not autism.