The first medical description of children on the spectrum was by a doctor in the late 1600s. Autism/spectrum behaviors have been documented, defined, discussed, and disbelieved ever since.
Pretty sure, though, that those kids on the spectrum back in 1700 and 1800 etc weren't caused by vaccination.
Also, it is more than possible to dx spectrum behaviors in the first 18 months if doctors follow up on concerns. Dd was 5 before she was dx, but there were symptoms there from birth. Before she had MMR there were symptoms that could and should have been picked up on. Unfortunately, the NHS doctors who noted and wrote about her symptoms didn't then flag up that there were possible concerns and refer her, they simply said that her hearing was fine and made no further comment.
I could look back at when she had her MMR and comment that her behavior became more noticeable soon after. It did. But not because of the MMR, just because of developmental stages. Correlation and causality are very different things.
And yes, she inherited her condition and was born with it, which is the science behind how/why she has this condition.