Since your disability basically cut off old career, they have to consider your ability to change careers, they call this vocational adjustment. At 55 they basically say its going to be harder to be harder to accomplish that and still get a pay off that makes it worth it.
At 45-49 and 50-54 and again at 55 they have different categories where they make assumptions about your physical capabilities. They amount to making it easier to prove that you can't do certain types of work. Still part of the bigger picture of skills and education though, and may not be a factor if you can already prove you have work restrictions. It tends to amount to those being under 45 being viewed critically when they say they can't do physical work though.
TLDR; age 55 is preferred.
