Yes, anyone of legal age can apply to become a member of Dignitas and thus for their help in assisted suicide. Of course there are fees. I don't know what they define 'legal age' as.
They turn down people with mental illnesses often. It's a lot harder to get approval for euthanasia with mental illness because you have to be able to show it's a rational decision and not 'just' a symptom, and the psychiatrist is subject to substantial scrutiny for doing it among other things, so approving it can create problems for them. But people have died with Dignitas for all kinds of reasons not limited to physical illness.