According to Legal Zoom, as many as 55 percent of American adults do not have a will or other estate plan in place. This number has remained fairly steady from 2000 to the present, although the number of American adults who have advanced medical directives has increased. Those adults who do have wills are typically older—in the 1990s, more than 90 percent of the wills probated in the United States were made by those who were 60 years or older. California, like all states in the U.S., has specific laws and rules regarding what makes a will legal in the state.