OK, she can apply for SSI immediately. SSI is not social security, but it is a program for the disabled poor, and can provide up to $735/month, currently. Some states also add a supplement amount to that $735.
Any other income she gets, including the value of shelter if gifted to her, will count againt that $735. If she lives with you, then you want to make sure you start to have a renting arrangement with her, where you charge her the proportionate share of mortgage or rent, plus basic utilities and food, so that she is not deemed to have her shelter given to her, because then they will knock her benefit down by about 1/3, since that part for shelter is no longer warranted. Consider the rent arrangement, so that amount will still come in the household, in the form of going to her as part of the 735, then X amount (say, for instance, you figure the math to be $500 for her), going from her to you. You can always use it in acceptable ways FOR her, that will not impede her SSI full benefit amount.
As for her future: She can remain on SSI indefinitely , so long as disabled and the very strict rule on having extremely small savings and no real income. By getting her on SSI now, at 18, you can keep those medical records and proof safely filed at home, so one day, when her legal parent(s) either retires, become SS disabled, or passes away, she can then file for Social Security Disabled Adult Child Benefit (Child's Disabled Benefit) based on that parent's work record. That will get her off of SSI and onto Social Security DAC, which is a little less strict and she will have more options in life, including being allowed to inherit money (which she can't safely do while on SSI, ergo, it is beneficial to get Social Security if possible). She has to prove only that she was disabled before age 22, which will be easy because she will already have proven that for SSI benefit, at 18.