§ 7. Voidable marriages. A marriage is void from the time its nullity is declared by a court of competent jurisdiction if either party thereto:

1. Is under the age of legal consent, which is eighteen years;

2. Is incapable of consenting to a marriage for want of understanding;

3. Is incapable of entering into the married state from physical cause;

4. Consent to such marriage by reason of force, duress or fraud;

5. Has been incurably mentally ill for a period of five years or more.