To qualify for disability benefits as an adult--which is at any age over 18, you must demonstrate to the satisfaction of the Social Security Administration not only that you have a disability with a physician's diagnosis, but more importantly that your disability and related conditions prevent you from performing Substantial Gainful Activity (SGA). SGA is defined (2018) as earning $1,180 in gross monthly income (may be higher if you are blind). That's about 36 hours/week at my home state of Illinois' minimum wage of $8.25/hour. However, it will not be enough to point to the fact that that you have never yet made that much money. Social Security has to believe you couldn't, even if you had the opportunity.
This means that you must bring to your intake interview a boatload of documents detailing specifically why and how your capacity to work is limited. You might even want to use a highlighter or Post-it® arrows to flag exact words and phrases. If you have worked, bring performance reviews. If you are still in school, bring your individual education plan (IEP) and the supervisor's or job coach's notes from your work experiences. If you've volunteered, get letters from the volunteer coordinator. But insist that the person be honest about your limitations rather than nice.
Social Security needs to know if you cannot stand, sit or walk for very long or if you cannot lift very much weight or use your hands to full capacity. You need to tell them if you cannot handle electronic buzz, loud noises, bright or flashing lights or beeping timers. If you need prompts to complete multi-step tasks, or if you cannot make change manually or if you get anxious during the lunch hour rush, they need to know. If you use a communication device, or have speech apraxia or if your hearing aids don't resolve voices in a crowd or if the intensity of your vision prescription makes it hard to work with screens, tell them. If for any physical, mental or emotional reason at all you cannot work that 36-hour week you need to find a way to show it in writing.
Social Security and Medicaid have a whole slew of work incentives to help youth and adults with disabilities work and earn up to their full potential, while still accessing the benefits that they need for support. The paradox is that to access these incentives, you must first prove your eligibility for the benefits, which in turn entails showcasing all the reasons that you cannot work very much. Having a medical or psychological diagnosis from a licensed physician is necessary, but not sufficient to be eligible.
If you or someone you advocate for has a real disability, be brutally honest about the limitations attributable to that disability. Bring as much specific evidence as you can find to document why you cannot, even with reasonable accommodations, earn to the level of SGA. Once you qualify for disability benefits, there are work-arounds to help you keep them even as you start to develop your capacity and earn more.