A prediction on the age of thick discs as a function of the stellar mass
  of the host galaxy
  One of the suggested thick disc formation mechanisms is that they were born
quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs
formed slowly within them from leftovers of the turbulent phase and from
material accreted through cold flows and minor mergers. In this letter, I
propose an observational test to verify this hypothesis. By combining thick
disc and total stellar masses of edge-on galaxies with galaxy stellar mass
functions calculated in the redshift range of $z\leq3.0$, I derived a positive
correlation between the age of the youngest stars in thick discs and the
stellar mass of the host galaxy; galaxies with a present-day stellar mass of
$\mathcal{M}_\star(z=0)<10^{10}\,\mathcal{M}_\odot$ have thick disc stars as
young as $4-6\,{\rm Gyr}$, whereas the youngest stars in the thick discs of
Milky-Way-like galaxies are $\sim10\,{\rm Gyr}$ old. I tested this prediction
against the scarcely available thick disc age estimates, all of them are from
galaxies with $\mathcal{M}_\star(z=0)\gtrsim10^{10}\,\mathcal{M}_\odot$, and I
find that field spiral galaxies seem to follow the expectation. On the other
hand, my derivation predicts ages that are too low for the thick discs in
lenticular galaxies, indicating a fast early evolution for S0 galaxies. I
propose the idea of conclusively testing whether thick discs formed quickly and
in situ by obtaining the ages of thick discs in field galaxies with masses of
$\mathcal{M}_\star(z=0)\sim10^{9.5}\,\mathcal{M}_\odot$ and by checking whether
they contain $\sim5\,{\rm Gyr}$-old stars.
