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One of the suggested thick disc formation mechanisms is that they were bornquickly and in situ from a turbulent clumpy disc. Subsequently, thin discsformed slowly within them from leftovers of the turbulent phase and frommaterial accreted through cold flows and minor mergers. In this letter, Ipropose an observational test to verify this hypothesis. By combining thickdisc and total stellar masses of edge-on galaxies with galaxy stellar massfunctions calculated in the redshift range of $z\leq3.0$, I derived a positivecorrelation between the age of the youngest stars in thick discs and thestellar 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 asyoung as $4-6\,{\rm Gyr}$, whereas the youngest stars in the thick discs ofMilky-Way-like galaxies are $\sim10\,{\rm Gyr}$ old. I tested this predictionagainst the scarcely available thick disc age estimates, all of them are fromgalaxies with $\mathcal{M}_\star(z=0)\gtrsim10^{10}\,\mathcal{M}_\odot$, and Ifind that field spiral galaxies seem to follow the expectation. On the otherhand, my derivation predicts ages that are too low for the thick discs inlenticular galaxies, indicating a fast early evolution for S0 galaxies. Ipropose the idea of conclusively testing whether thick discs formed quickly andin 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 whetherthey contain $\sim5\,{\rm Gyr}$-old stars.

A prediction on the age of thick discs as a function of the stellar mass of the host galaxy