Abstract: An anti-aging additive composition and an operating method to control the quench oil viscosity in an ethylene production plant are described, capable of raising the yield of said plant through a control of the quench oil viscosity related to a significant increase in the operation temperature of the plant fractionating column, said composition comprising an effective quantity with respect to the aging of said oil of at least one component selected in the group constituted by alkylated phenols, alkylated biphenols, diphenols, alkylated diphenols, aromatic amines and nitroxides, precursors and mixtures thereof.