If a person hasn't earned too many quarters working, they may be eligible for SSDI under their parents accounts, instead of SSI. Such SSDI is generally a few hundred dollars a month greater than SSI, and SSDI includes forms of Medicare for disabilities at any age.
Getting SSDI with no, or few, work quarters is possible by having sufficient documentation of early impairments once employment becomes difficult to nearly impossible in adulthood. Too many work quarters will exclude coverage under a parent's account. And, Social Security demands application under SSDI before, or along with, application for SSI. Without making a good case for SSDI, more quarters have to be earned before re-application, and the possibility of drawing on a parent's account is lost, once the application is denied and not successfully appealed to higher levels, after age 22. Even if SSI is awarded, a denied but still pending SSDI claim can still be appealed to higher levels.
http://www.ssa.gov/dibplan/dacpage.shtml
(similar claims possible with many nebulous exceptions in every direction)
The legal argument is that a disabling impairment started before the age 22, not that the application for such SSDI has to be made before the age 22 (be ready to appeal to the federal courts when the SSA renders an unwanted determination).
I was finally forced to apply for SSI while I was claiming that I was otherwise qualified for employment with "reasonable accommodation" as an FDIC Bank Examiner in 1987 under their outstanding scholar program. I completed university magna cum laude with the paradox that anything boring and mundane always gave me a migraine, and I needed intellectually challenging employment. It was 12 years before the Supreme Court finally established that "ADA disabled but employable with reasonable accommodation" and "Social Security disabled" were not necessarily mutually exclusive concepts, as the 9th Circuit Court was then holding, but with me, I was held "too disabled" for Rehab services, but not proven "enough disabled" for discrimination protection under the Rehab Act. I was also told that I was "too smart to be a cop", even though I was stupid enough to sue various offices of the federal police for employment discrimination, thinking before then that "Catch-22" was a work of fiction. While the FDIC argued during administrative remedies that my disabilities, that qualified me for the SSI "favorable" decision from the SSA for benefits, disqualified me from being otherwise qualified for the FDIC job, the FDIC then argued in federal court that I couldn't provide a scintilla of evidence that I suffered from any handicapping impairment (by hindsight, maybe they didn't discriminate(?), they just wanted agents who would enjoy watching innumerable trillion dollar financial bubbles inflate everywhere without concern).
Tadzio