These services can be accessed by any student who has been deemed eligible by Social Security. Medicaid can be used to utilize adult services as a secondary insurance for your child. At the age of 18 it is important to apply for this.
For more information on SSI Eligibility and Application Process:
Social Security for Children Under 18
Social Security for Adults (Over Age 18)
Social Security Frequently Asked Questions (FAQ)
SOCIAL SECURITY RED BOOK PDF - http://www.ssa.gov/redbook/
For information on Medicaid:
https://www.health.ny.gov/health_care/medicaid/how_do_i_apply.htm