My parents say that when I'm 18, I can't move out because of my mental disorders. Is that true or not?

Have you been formally diagnosed? Could you live on your own?

It depends on the level of mental disorder...

If you are diagnosed as not capable of caring for yourself, they can control you.

What is the mental disorder

It also depends on whether your parents have been assigned as guardians. Basically, at 18 you are an adult and can do what you want. Unless a court rules otherwise.