I can't tell if the OP's daughter has MH problems or not.
However, if she has, it may be more complicated than depression. While there may be plenty of people with MH problems, living in their own home, equally if you look at a report say by Crisis, you will see that its estimated 2/3 of single homeless people on the streets have undiagnosed personality disorders; and the rate of psychosis is higher than in the normal population. Ditto, there are plenty of people in jail with mental health problems. It is estimated that 50% of male suicides are undiagnosed personality disorders.
We have one disabled daughter, with complex learning difficulties. Our other daughter had OCD and social anxiety at school - both aspects of a generalised anxiety disorder. This was not surprising in view of the family circumstances. She had CBT, which did nothing for her. As she was suicidal just before her A-levels, I made an emergency appointment for the GP, and got her reluctantly to a go. She got through her A-levels on anti-depressants and took a gap year. She seemed to have plenty of friends, a social life and got a bf. She started at uni, and just after the relationship with the bf broke up, for reasons beyond the control of either of them. More depression, until we found out she had spent 2 weeks researching on the internet how to commit suicide. I made another emergency appointment with the GP and got her to come with me.
She ended up being diagnosed by a psychiatrist with mixed personality disorder - elements of dependent personality disorder and borderline personality disorder. The three causes were:
- the trauma of seeing what went on with her disabled sister
- feeling neglected by us, as so much of our attention went on the sister
- sexual abuse by the bf - he had tried to force her to do everything. She put up with it, because she wanted someone of her own, to care for her
It is not for me to say that the OP's daughter has personality disorder, but it is more difficult to deal with than just depression. Our daughter went on to be taken by the emergency services either to A & E or an acute mental health unit for suicidal behaviour 15 times in the next 18 months.
I understand that if you have one child with special needs, there can be an intense desire for the other siblings to be "normal" - to leave home in their early 20s, get a job, married whatever, because you think "I have enough to do for the special needs one; I need the others to be normal!" Unfortunately, life is not always like that. If another one has significant mental health problems, they need just as much support from their parents - and telling them to stand on their on two feet, is a complete waste of breath! Abandonment can be a major trigger.
One example, we told our daughter who was being sent to an acute daily treatment unit, run by mental health services, where they had lectures. She did not like one of the lecturers and did not want to go the next day. We told her calmly, she wanted treatment for her condition, and should give it a good go, and then she could say that it did not work for her. It was only sitting in a lecture; not like being friends. At this, she went berserk, physically attacked me - husband had to restrain her. Then she stormed out of the house, to the railway station to attempt suicide - they had to do an emergency stop of a train, and the police took her to the local acute mental health unit. The police have had to jump down onto the tracks before to drag her off; or had to chase after her on the streets at midnight, while she is running around, in the process of taking an overdose, on top of neat vodka!
Even though people like her, do end up living in a flat, the reality is that often the family have to run their home for them - at uni, she was living on Weetabix or not eating at all; spending all her money on clothes and then couldn't pay the bills!
It really is not necessarily, just a case of depression; and saying things like "I had a job and my own house at 23". It is about as helpful, as telling someone in a wheelchair "Well, I can walk, so you should be able to!"