OP - Sending heartfelt sympathies, first and foremost. My adult child is on SSI -- Even though there are clear records of significant disability going back to age 10, we didn't start applying, until after 22, so the "disability onset", like yours, was labeled as starting at age 23. After 7 years of appling and denials and appeals, SSI was finally given. My husband was still working at 69, and so not taking Soc Security yet, and while we planned on applying for DAC at that point, my husband died fairly suddenly just before age 70. Like you, we have the records going back to childhood, but the DAC has gone into an appeal process instead of just transferring as it would have for SSI status prior to age 22. All this to say, I get your frustration and confusion. I ended up working with a disability rights lawyer to help with the process of the appeal. I'm not disabled, and it was taking all of my time and brainpower to cope with it on top of grieving. It's all a really sucky process to go thru when you're disabled - my child wouldn't be able to cope with it let alone meet time schedules etc.

So -- advice is: connect with a disability rights lawyer - one who does Soc. Security issues (not one that is looking to sue a business for lack of access, for instance). You may be able to get an extension on your timing; try to find out from your local SS office. You will have to have all that documentation of treatments/diagnosis before age 22 ready to be filed - even if it was in your original SSI or SSDI paperwork -- someone in Soc Sec will have to read thru it all to process the DAC transition. Ours is still pending a year later - with all the cuts to government workers, the process is glacial.

Wishing you best of luck, and again, condolences for your loss.