R.S.
I am pretty sure there is no law on the legal age. It is more of a use your own judgement. But, that is sometimes hard. I would say that if your son know how to take care of all her special needs (meds, moods, etc) than it would be fine. Another thing to consider when making the decision is how far away are you in the event of an emergency (or something your son is having trouble dealing with) or is their a neighbor or friend nearby that could be there quickly if needed? Sounds like a stressful decision but I am sure you will know what to do soon.