(2) Infants, etc., Defend by Guardian Ad Litem. - In actions or special proceedings when any of the defendants are infants or incompetent persons, whether residents or nonresidents of this State, they must defend by general or testamentary guardian, if they have any within this State or by guardian ad litemappointed as hereinafter provided; and if they have no known general or testamentary guardian in the State, and any of them have been summoned, the court in which said action or special proceeding is pending, upon motion of any of the parties, may appoint some discreet person to act as guardian ad litem, to defend in behalf of such infants, or incompetent persons, and fix and tax his fee as part of the costs. The guardian so appointed shall, if the cause is a civil action, file his answer to the complaint within the time required for other defendants, unless the time is extended by the court; and if the cause is a special proceeding, a copy of the complaint, with the summons, must be served on him. After 20 days' notice of the summons and complaint in the special proceeding, and after answer filed as above prescribed in the civil action, the court may proceed to final judgment as effectually and in the same manner as if there had been personal service upon the said infant or incompetent persons or defendants. All orders or final judgments duly entered in any action or special proceeding prior to April 8, 1974, when any of the defendants were infants or incompetent persons, whether residents or nonresidents of this State, and were defended therein by a general or testamentary guardian or guardian ad litem, and summons and complaint or petition in said action or special proceeding were duly served upon the guardian or guardian ad litem and answer duly filed by said guardian or guardian ad litem, shall be good and valid notwithstanding that said order or final judgment was entered less than 20 days after notice of the summons and complaint served upon said guardian or guardian ad litem.