Every corporation must be managed by a board of directors or trustees, all of whom must be at least 18 years of age. Every corporation must have at least one director or trustee. All corporations may provide in their articles or bylaws for a fixed number of directors or a variable number of directors within a fixed minimum and maximum, and for the manner in which the number of directors may be increased or decreased. Unless otherwise provided in the articles, directors need not be members. The articles or bylaws may provide that some or all the directors or trustees must be chosen by specified persons or by public officials.