DATE AT WHICH AUTISM RATES BEGAN TO DECLINE
According to the California DDS, "As of December 2002, estimated
prevalence reached a high at 31.2 per 10,000 for birth year 1997."
(2003 Calif Autism Rept P. 10). http://tinyurl.com/9ztpo  This means
autism rates began to decline for the children born early 1998.  The
children would have been receiving their 18 months shots in or after
July, 1999.

DATE AT WHICH THIMEROSAL WAS REDUCED
According to the CDC, "in July 1999, the Public Health Service (PHS)
agencies, the American Academy of Pediatrics (AAP), and vaccine
manufacturers agreed that thimerosal should be reduced or eliminated
in vaccines as a precautionary measure." http://tinyurl.com/8dkxx

It's hard to conceive better timing.