You can't wait until you need a living will to create it.
One of the more difficult things in life is thinking about the fact that it will end and this prevents many people from considering a living will or other advanced medical directives. It is tough to consider but there is the possibility that you cannot make your own decisions in the future. However, as Psychology Today points out in "Putting It in Writing," it is important to think about and get a living will.
The biggest benefit to making the effort to get a living will is you cannot do it later. You cannot put off getting a living will until you need it. By definition, when you actually need the document you are not legally able to get it.
Thus, if you want to have a say in what measures doctors will take to prolong your life when you are unable to tell them your wishes, then you need to do that ahead of time. There is also a benefit in that having the document can give you peace of mind that you have taken the appropriate steps to ensure your wishes will be carried out.
An estate planning attorney can help with a living will or other advanced medical directives.
Reference: Psychology Today (April 7, 2016) "Putting It in Writing"