It may be an SSI payment but the application uses the SSDI disability criteria. Adult SSI is financial need based payment. As I read the regulations, I was appalled to see that someone could qualify for SSDI based SSI without any work history whatsoever. Maybe the SSA people use the terms interchangeably.

I just looked up the SSA website. I was correct with the first statement. The web site says
<SSI rules about disability

Your child must meet all of the following requirements to be considered disabled and therefore eligible for SSI:

* The child must not be working and earning more than $1,000 a month in 2011. (This earnings amount usually changes every year.) If he or she is working and earning that much money, we will find that your child is not disabled.
* The child must have a physical or mental condition, or a combination of conditions, that results in “marked and severe functional limitations.” This means that the condition(s) must very seriously limit your child’s activities.
* The child’s condition(s) must have lasted, or be expected to last, at least 12 months; or must be expected to result in death.

If your child’s condition(s) results in “marked and severe functional limitations” for at least 12 continuous months, we will find that your child is disabled. But if it does not result in those limitations, or does not last for at least 12 months, we will find that your child is not disabled.>

Those criteria are the same or very similar to the SSDI criteria. This is not an I'm right, you're wrong. It is more that SSA is a very confusing system with overlaps in their regs and terminology.

To make matters more confusing, SSA says
<Social Security Disability Insurance (SSDI) benefits for adults disabled since childhood

The SSDI program pays benefits to adults who have a disability that began before they became 22 years old. We consider this SSDI benefit as a “child’s” benefit because it is
paid on a parent’s Social Security earnings record.
For a disabled adult to become entitled to this “child” benefit, one of his or her parents:

* Must be receiving Social Security retirement or disability benefits; or
* Must have died and have worked long enough under Social Security.

These benefits also are payable to an adult who received dependents benefits on a parent’s Social Security earnings record prior to age 18, if he or she is disabled at age 18. We make the disability decision using the disability rules for adults.

SSDI disabled adult “child” benefits continue as long as the individual remains disabled. Your child does not need to have worked to get these benefits.>

Sage's daughter would be in this Disabled since childhood category. It's a confusing mess.