I don't think being young matters much. I'm 30 and I'm currently on SSDI for physical disabilities and have been for 4 years. I did get approved the first time around. I know that's not uncommon. I have kind of a different perspective. I'm a physician and I did a lot of SSDI evals. Being young doesn't really matter, I saw just as many young people as older people get approved.
I disagree with the idea of getting a lawyer initially. I would wait until and if you get denied before getting an attorney. They'll take your case on a contingency, but they will get paid eventually. They'll take a significant portion of your monthly amount, if you win, for a long period of time. And I have nothing against attorneys. My father's an attorney.
I think it's more important for you to develop a strong relationship with a physician who'll go to bat for you. Like everyone said, you need significant documentation of a severe and permanent disability to qualify.
Good luck.