Its not that easy.
The SSI process is a 5 step process (for adults)
The first step is determining if you meet the financial eligibility requirements for the program. For SSI this is an income sensitive test. Income (for both adults and for children cases) is not just what the individual makes, but also the assets and income of all people living in the household with the person. For adults they will also include any support that person is getting (for example your living at home rent free, the average rental room rate would be given to the person as income, unless they show proof that they have to repay this with any backpay). For children, there is what is called a deeming process by which they give some of the parents income to the child (its not something one can easily sit down and figure out). For an adult, if you have just about any income in the family or more then $2,000 in assets, your not going to be eligible.
The 2nd step is then to determine if the person has a medically severe disability. SSA has what are called listings. These listings are specific conditions at a specific severity which if you exactly meet these requirements, your automatically eligible for disability benefits (given that you have passed the test at step 1). You can also argue that your condition equals a listing (the equivalent to one of the listings). If you don't meet a listing, then you move on to the next steps.
The next steps look at whether or not your condition while severe, interferes enough with your life to (for adults) rule out all work (at the specified work level based on your age) or for kids severely disrupts at least two major life activities.
ANY disability can be eligible based on how it affects a person.