Re: Five year rule?
Work credits are "rolling", so they age off. After 5 years of not working, there wouldn't be enough to "qualify".
It has to do with when your disability started. If you can medically prove it started on a date while you were covered by the SSDI (had enough work credits) you can qualify for SSDI (Social Security Disability Insurance). It's insurance you paid into while working. When you aren't working, you aren't "paying premiums". Otherwise, you have to be very poor with few assets to qualify for SSI (Supplementary Security Income).
Basically, you want your onset date to be a date during the time you were covered by SSDI. (I filed 7 years after I quit work, proved my onset date was during the correct timeframe, and won my SSDI case.)