  Reconstructing the mass assembly history of the Milky Way relies on obtaining
detailed measurements of the properties of many stars in the Galaxy, especially
in the stellar halo. One of the most constraining quantities is stellar age, as
it can shed light on the accretion time and quenching of star formation in
merging satellites. However, obtaining reliable age estimates for large samples
of halo stars is difficult. We report published ages of 120 subgiant halo stars
with highly-radial orbits that likely belong to the debris of the
\textsl{Gaia-Enceladus/Sausage}~(GES) galaxy. The majority of these halo stars
are old, with an age distribution characterized by a median of 11.6~Gyr and
16$^{\rm th}$(84$^{\rm th}$) percentile of 10.5~(12.7)~Gyr. However, the
distribution is skewed, with a tail of younger stars that span ages down to
$\sim6$--$9$ Gyr. All highly-radial halo stars have chemical and
kinematic/orbital quantities that associate them with the GES debris. Initial
results suggest that these intermediate-age stars are not a product of mass
transfer and/or stellar mergers, which can bias their age determination low. If
this conclusion is upheld by upcoming spectro-photometric studies, then the
presence of these stars will pose an important challenge for constraining the
properties of the GES merger and the accretion history of the Galaxy.
