If your impairment continues after age 18, then depending on the nature and extent of your impairment, informal arrangements can be made where family, friends, neighbours, health professionals and so on provide you with assistance, support and information so that you can make your own decisions and communicate your decisions to others, without having to involve the courts and the legal system.
There are also more formal systems for decision making: see above and see the chapter “Decision making and powers of attorney”.
The “supported decision making” approach is endorsed by the UN Disability Convention (see in this chapter “Rights that are recognised internationally: The UN Disability Convention” and “Supported decision making: Using New Zealand law to put in place a supported decision making approach, like a ‘circle of friends‘”).