Animal research, which plays a critical role in drug discovery, has been of central importance for the production of background knowledge for clinical investigations and the optimization of procedures prior to human trials. An important feature of in vivo modeling is matching of the age of animals used in preclinical research to the age of peak incidence for a disease state in humans. Indeed, it is clear that animal models must match humans at a similar stage of life to be used as successful platforms for clinical translation, as many conditions that are targeted for drug treatment are age dependent. Valid age matching is critical for successful use of animal modelling in biomedical research in general and, in particular, drug discovery. It is important for researchers to understand that relative ages are different depending upon the stage of life. Therefore, one has to determine the relevant age under investigation and what factors are being analyzed. For this, special attention is needed to verify the phase of life of the animal (in days) and its correlation with the age of humans (in years). In this article, we focus on an even more specific issue: the degree to which similarities between species reflect fundamentally similar biological processes at a particular stage of development or age.