The latest CDC figure for autism--1 in 88, is from 8 year olds in 2008. So they were born in 2000, before thimerosal was removed.
But it will never be clear cut, even 10 years from now, because as thimerosal was being removed from most pediatric vaccines, the CDC began promoting flu shots (no preference for mercury free doses) to pregnant women and babies starting at 6 months. So the exposure to thimerosal began at an earlier age (as developing fetuses). In 2009 and 2010, many pregnant women got injected with 50 mcg mercury (25 mcg from the regular flu shot and 25 mcg from the H1N1 flu shot).