I applied and was on SSDI for 2 years before turning 66 and going on SS. The process is not an easy one and the older you are the easier it is to qualify. The basic requirements are that you cannot return to your job and are fully disabled and expect to be for at least a year. The standards are relaxed over 55.
SSDI is bascially a process of denial however. Some conditions like ALS or stage 4 cancer will be approved quickly but most AI diseases are in a gray area. SS stats are that 65% of 1st applications are turned down. I was a quadriplegic (I have lupus and polymyositis) with little hope of ever walking again and they just said I will be OK. 2nd apps are even less likely to get approved and only 10-15% get approved. The 3rd app goes to an administrative law judge and 65% get approved but the wait to see an ALJ in some states is 2-3 years.
I applied 3 times and the 3rd time the ALJ did not even hear my case and approved it in 2 weeks. My condition had improved but I was still unable to work.
The application process is one of denial and it seems they look for mistakes and anything to deny applications. I did get an attorney who specializes in SSDI for the 3rd app and he made sure everything was in order. He also pointed out that despite my condition and age (63 at the time) mistakes were made in the application and letters from my well intending docs. These lawyers take cases on contingency and by law can only charge 25% of retroactive compensation up to a max of $5300. Talking to one is a good test of wether or not your case has merit. They will not take a case they think will not be approved.
Others on this site have applied so ask your questions. It is possible to get SSDI for lupus but getting approved for almost any illness is usually a fight so prepare yourself. It pays to be persistent. You will get whatever your full retirement SS income would be and younger people will probably be reviewed for any changes or improvement in their condition.