There is autism, and there are autism-like conditions. Heavy-metal poisoning will produce "autism" in an otherwise-normal child, and chelation therapy will get rid of the heavy metals and the "autism." But let's not confuse that with the genuine autism that is based on the presence of certain aberrant genes plus certain prenatal triggers.
Actually, while chelation therapy can reduce heavy-metal blood levels in a child who has been exposed to such metals, whether or not it will 'get rid' of such is very much a factor of how high a level the child had prior to it being identified and chelation begun. Heavy-metal poisoning can
(not 'will') produce autistic symptomatology; however, it can also produce a variety of other emotional and/or intellectual consequences, either rather than or in addition to autistic symptoms.
Vaccines as a causative agent for autism is generally discredited in the medical literature, however there are still significant numbers of people who choose to believe it is either the or, at very least, a cause. There are also any number of other theories as to the cause.
The so-called autistic spectrum is broad, probably much too broad - leading to the types of behavior and issues that several folks have raised - and to such disparities of presentation as that depicted by Dustin Hoffman versus the life lived by Temple Grandin. The initial descriptions of Early Infantile Autism by Leo Kanner in the late 1940s involved such a highly defined synptom cluster that diagnosis could be accomplished by a checklist. But, in retrospect, it's clear that the patient population which initially came to Dr Kanner's attention were, indeed, almost all in what would subsequently be recognized as the condition's truest and most severe form. In that state, it can certainly be said to be rarely, if ever, curable. In fact, it's doubtful if true autism can ever be completely cured. What can be hoped is to mitigate its effects.
As regards the 18 month of age issue - the association with vaccines was originally formulated because of that age factor and that vaccines were fairly routinely admnistered at about that age. However, Kanner identified that as a common age at which parents and others first started to be aware of the symptom cluster. In retrospect, however, under careful questioning, parents and other caregivers were often found to describe earlier signs that were not noted as important at the time (and would not have been to an unskilled observer) or were written off to immaturity, etc.
Finally, although multiple instances in among sibings are not unknown, single children - regardless of his or her birth order - are likewise not uncommon.
Neil (once involved in the assessment of autistic children, now the Dad of a child with autistic overlay, tentatively diagnosed as Aspberger's Syndrome)