Being diagnosed should not prevent you from making an advance directive as long as you are in full possession of your mental faculties. That’s how it works in Spain at least. In certain cases this may involve a medical evaluation.
We had some issues in the family with this, but yes, it might be because the diagnoses was already (perceived to be) too late. I would recommend doing it young if it's a familiar/genetic issue. If not, of course you cannot do that, so you try what you can.