  The Milky Way was shaped by the mergers with several galaxies in the past. We
search for remnant stars that were born in these foreign galaxies and assess
their ages in an effort to put upper limits on the merger times and thereby
better understand the evolutionary history of our Galaxy. Using 6D-phase space
information from Gaia eDR3 and chemical information from APOGEE DR16, we
kinematically and chemically select $23$ red giant stars belonging to former
dwarf galaxies that merged with the Milky Way. With added asteroseismology from
Kepler and K2, we determine the ages of the $23$ ex-situ stars and $55$ in-situ
stars with great precision. We find that all the ex-situ stars are consistent
with being older than $8$ Gyr. While it is not possible to associate all the
stars with a specific dwarf galaxy we classify eight of them as
Gaia-Enceladus/Sausage stars, which is one of the most massive mergers in our
Galaxy's history. We determine their mean age to be $9.5^{+1.2}_{-1.3}$ Gyr
consistent with a merger time of $8$-$10$ Gyr ago. The rest of the stars are
possibly associated with Kraken, Thamnos, Sequoia, or another extragalactic
progenitor. The age determination of ex-situ stars paves the way to more
accurately pinning down when the merger events occurred and hence provide tight
constraints useful for simulating how these events unfolded.
