For income tax purposes, disabled means "unable to perform gainful work." So it depends on the degree of autism. Gainful work is work done for at least minimum wage. Work in a sheltered workshop or rehab facility does not count, but if a person can work a minimum wage (or more) job with reasonable accommodations, they are not disabled for income tax purposes.
Also, for children under age 13, checking the box does not change anything on the tax return because there are no specific tax benefits for disabled children. At age 13 and older, being disabled makes the child eligible for the child care credit (if you have to pay for child care because they can't care for themselves), and over age 18, being disabled may allow you to claim an adult child as a dependent even though they have aged out of the normal dependent rules.
