I have a dependent who is 18 years old, but is disabled. How do i file for him on turbo tax?

If you have a medical provider decision that they are disabled be sure to check that box when you claim them as a dependent. If social security is their only income there’s no filing requirement for the child. 

You may be able to claim a relative over 18 who is an SSI or Social Security recipient as a dependent and receive the Credit for Other Dependents if you meet the test for a qualifying relative per Table 5 from IRS Publication 501 at this link.

See here for more information from the Social Security Administration on SSI. The Gross Income of an adult SSI recipient would affect SSI benefits but not support received from a relative. 

See this article for more information on qualified dependents. Enter dependent information in the Personal Info section of TurboTax.