Like almost all answers in the law, it depends. The Social Security Administration (SSA, www.ssa.gov) considers a number of factors such as age, education, years worked, type of work, whether your skills are transferrable and salary received. They also consider evidence of physical impairments and evidence of mental impairments. I tell my clients there is no "silver bullet," as the saying goes," because there are intangible things about a case that can turn a case.
If the case goes to hearing, an administrative law judge (ALJ) will issue a written decision. In cases that aren't as clear cut I often see the judge rely on testimonial evidence. This means how the client came across at hearing. In other words, was the claimant credible? Fortunately the "human element" is still relevant, and judges, who are people too and trying to do the right thing, can be--and often are--swayed by a compelling story.
So I can't say who gets disability, but I can say when I've got something to work with to create a case that has a fighting chance.