One of the requirements of a valid will in California is that the creator of the testamentary document is of sound mind when they sign their name to the will and make it official. A person who lacks the mental capacity to understand the terms of their will likely would not have the soundness of mind required by law to execute it, and to this end, if a person does not have a will when they lose their capacity to create one they may not be able to prepare one to protect their estate.
For example, consider a person who is diagnosed with dementia. At its onset, the disease may not rob the person of the soundness of their mind, but they may begin to exhibit some of the very earliest stages of the medical condition. If they elect not to create their will or any other estate planning documents, and assume that they will be able to so do at a later date, they may not be able to if their disease progresses and robs them of their intellectual capacity.
Wills are not the only documents individuals should prepare before they might lose their capacity to do so. A power of attorney document can be immensely important to have in place, as well as a medical directive, that may allow another person to make decisions for the incapacitated person. When these documents are not prepared, and a person cannot manage their own financial and medical needs, their loved ones may need to work through guardianship proceedings in order to gain the right to help them.
It is never too early to create an estate plan and a person is never too young to have one on place. Readers without estate plans are encouraged to take active steps to have their testamentary wishes put to paper to protect them in the event they later are unable to take such actions.
Source: nwitimes.com, "Estate Planning: Create the plan before you need it," Christopher Yugo, Jan. 14, 2018