I used to be a claims adjudicator in the US. A diagnosis alone isn't enough. It's all about whether you function well enough to work. You'd need to have 2 out of the following four items 1) marked restrictions in activities of daily living (can't drive, or use public transport, prepare simple meals, remember to bathe, et.c) 2)marked restrictions in maintaining social functioning (can't accept criticism from bosses without melting down or deal with coworkers appropriately)3)Marked difficulties maintaining concentration, persistence or pace (can't remember how to do simple, repetitive work or do it in a timely fashion) 4)repeated episodes of decompensation (this means you have been hospitalized for mental issues for several weeks out of a year). The items in parentheses are just examples so that you would know the level of impairment that is meant by marked...it is by no means limited to that. It just takes a lot for them to consider one of these categories marked. Most who are higher functioning pretty much don't have a shot.
If you don't have a parent receiving social security, at 18 you most likely would only get SSI if approved medically (which caps out at about $600 a month), since to get Disability, you have to have worked five out of the last ten years.