I'm working with a youth on SSI who is soon turning 18. When can he apply for SSI as an adult?
In general, a child/youth can apply for adult benefits the month before they turn 18.
There is a new exception for disabled youth transitioning out of foster care. Disabled youth receiving Title IV-E federal foster care benefits usually cannot become eligible for SSI until foster care payments have stopped. Even if the individual had been receiving SSI as a child, a new determination must be made under the adult rules once that person is 18. To help with this transition, SSA recently changed its rules so that an SSI application may be accepted from a youth in foster care up to 180 days* before foster care payments are expected to end.
For more details, see https://secure.ssa.gov/apps10/poms.nsf/lnx/0500601011!opendocument. The application could be completed using the regular SOAR Process because you will be using the adult applications. You can start collecting all of the youth's medical records and preparing the application materials. Then you would have the completed SOAR packet ready to go as soon as they are able to submit the application.