A major determinant of the fitness of biological systems is their ability to integrate multiple cues from the environment and coordinate their response accordingly. Yet, our understanding of combinatorial integration of multiple inputs and its age dependence is still limited. One of the well-studied examples of such regulation is catabolite repression - a phenomenon where preferred carbon source (e.g., glucose) represses the pathway required for the consumption of alternative carbon sources (e.g., galactose). As a model system, we study how yeast response to hundreds of environments with different carbon sources as a function of time and age. We found that, in contrast to the textbook view, instead of merely inhibiting galactose utilization when glucose is above a threshold concentration, individual cells respond to the ratio of glucose and galactose, and based on this ratio determine whether to induce genes involved in galactose metabolism. We investigate the genetic architectures that can result in a ratio sensing and derive the conditions in which the optimal switching strategy involves preparation and when it is changed from threshold-sensing to ratio-sensing. We characterize the ability of cells to respond to changes in carbon source as a function of age and show that there is a non-trivial relation between mortality rate and failure rate.