When either of the parties to a marriage shall be incapable, from want of age or understanding, of contracting such marriage, the same may be declared void by the district court, in an action brought by the incapable party or by the parent or guardian of such party; but the children of such marriage begotten before the same is annulled, shall be legitimate. Cohabitation after such incapacity ceases, shall be a sufficient defense to any such action.