The aim of this paper is to explain phenomena associated with bitumen aging and establish design patterns that contribute to optimizing the aging resistance of bituminous mixes based on the rational addition of calcareous fillers (hydrated lime and limestone filler). The potential benefits of using fillers to improve aging resistance are well-known. However, concepts related to the biphasic filler-bitumen system affecting some of the main characteristics of mixes are not always considered. In this paper, a new study procedure, “Universal de Caracterización de Ligantes” (UCL®), a method for haracterizing both conventional and polymer-modified binders developed at the Technical University of Catalonia, has been applied. In addition, rheological tests have been performed with the dynamic shear rheometer and non-routine procedures such as macromolecular analysis (e.g. gel-permeation chromatography and infrared spectroscopy). These techniques have provided greater insight into the causes and effects of aging based on the changes undergone by the bituminous binder. The obtained results show the advantages of incorporating calcareous fillers into bituminous mixes provided that filler content does not exceed a “critical concentration” determined by the type of filler and binder to be used. Moreover, some variations on the concept of critical concentration of filler in bituminous mixes have been found, notably those resulting from the degree of mix aging and use of polymer-modified bitumens.