Age probably makes a lot of difference for SSI, especially if your parents/guardians did not receive SSDI/SSI for you as a younger person. SSDI is Social Security and dependent on your (or a parent/guardian's) long term work history but if you have not worked much and are young, it can be hard for them to judge you permanently disabled as there's so many more years ahead and there's not much background, it seeming "sudden" like. Just like you need a history of work for SSDI and that not working out well over time, you need a history of disability over time for SSI, and trying to work, etc. and that not working out.