Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, but require comparison with stellar models. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting, till now primarily employed to study nearby galaxies. Accurate distances to individual Milky Way stars provided by the Gaia mission have allowed us to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced color-magnitude diagram of the kinematically hot local halo, whose blue sequence has been linked to a major accretion event. Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation, these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.