Age is a fundamental property of stars. It is an essential tool to understand many diverse phenomena in astrophysics, including the evolution of stars, planetary systems, and the Galaxy. However, age is currently the most poorly known property of a star, often to no better than 30-40% accuracy, which is not good enough.The ages of stars cannot be measured directly; they can only be determined by comparing age-sensitive observables with model predictions. Asteroseismology, the study of stellar oscillations, offers the unique opportunity to estimate the ages of stars to within 5-10% of their lifetime. Using state-of-the-art space observations (CoRoT and Kepler) of stellar oscillation frequencies combined with ground-based spectroscopy (e.g. APOGEE), I propose to uniformly determine accurate ages of thousands of stars with unprecedented precision. Building on my extensive experience in this field, I plan to develop and implement new asteroseismic diagnostics for a large number of main-sequence stars, subgiants and red giants. These new age determination methods are expected to be calibrated using stars in binary systems and clusters, and compared with classical methods.Uniform age determinations for a large sample of stars in different directions in the sky will greatly advance the study of stellar populations in the Galaxy. This project is ambitious, and success requires a dedicated approach from a competent team with the right resources and the right leader.