  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, being comparison with
stellar models (Soderblom 2010) required. Alternatively, age distributions can
be derived applying the robust technique of colour-magnitude diagram fitting
(Gallart et al. 2005), till now mainly employed to study nearby galaxies. The
new distances to individual Milky Way stars from the Gaia mission (Brown et al.
2018) have allowed us to use this technique to derive ages from a thick disk
colour-magnitude diagram, and from the enigmatic, two-sequenced
colour-magnitude diagram of the kinematically hot local halo (Babusiaux et al.
2018), which blue-sequence has been linked to a major accretion event (Haywood
et al. 2018, Helmi et al. 2018). 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 (Brook et al.
2012), 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.
