The effect of an interface structure and stress state on the long-term strength of polymer composites is investigated and related to the adhesion between the polymer and the second phase. The approach specifically accounts for the previous life-cycle steps of the material through the evolution of the size of the reinforcement phase and of the interfacial internal stress state. PET/SiOx multilayer films are used and found appropriate to tailor the interface interactions and introduce controlled compressive stresses in the interface region. Adhesion is characterized by means of micromechanical and thermodynamic complementary approaches. The long-term strength of PET multilayer composites aged below the glass transition temperature of the polymer is predicted from the failure mechanism of the interface. It is shown that reliable predictions require the knowledge of the evolution of the interface shear strength, interface internal stress, and particle size through the whole life-cycle of the composite.