Formation (lookback) time of the discs in our sample. The left panel shows an example of our definition based on the circularity parameter as a function of the stellar age of all stellar particles in a given galaxy (blue dots). We calculated the running median (black symbols) and their derivative (bottom panel) to find the time when the dominant and consistent formation of stars in circular orbits occurs, T_c. We note that by definition, T_c is in lookback time units where T_c = 0 means today. In the right panel we show T_c for all disc-dominated galaxies as a function of their virial mass at the time of T_c (individual symbols). The median value shows that mostly-disc objects settle into circular orbits slightly earlier than for the intermediate objects, but the scatter is large. Galaxies typically reach T_c late and when their host halo mass is larger than M₂₀₀ ∼ 3 × 10¹¹ M_⊙ (with one exception). Some of the oldest discs have been consistently forming stars since ∼10 Gyr ago.