A marriage is illegal and shall be void from the time its nullity is declared by decree, if either party thereto—
(1) is an idiot or a person adjudged a lunatic;
(2) has consented thereto by reason of fraud or force;
(3) is incapable, from physical causes, of entering into the marriage itself; or
(4) is under the age of consent, which is hereby declared to be 18 years of age.