Self-determination is the determination of one's own fate based upon a freely determined set of criteria. This does not mean people are free to do whatever they choose without ramifications for poor decisions. It does mean a person should be given the opportunity to succeed or fail in pursuit of their own happiness.
When emphasis is placed on the independent determination of choices the true value of self-determination is lost. No one is ever truly independent. Regardless of the presence or absence of a diagnosis, everyone is interdependent on others around them. The influences which shape who we are, come from surrounding people, whom we trust and past experiences.
Supporting a person with a disability includes providing that person with complete information, presented in a matter they can understand in order to make an informed decision.