[Si/Fe] versus [Fe/H] (panel a)), [Si/Fe] versus age (panel b)) and [Fe/H] versus age (panel c)) for the data adopted in this work from Adibekyan et al. (2012) and Haywood et al. (2013). In each panel, the orange points represent thick-disc stars, green points “outer” thin-disc stars and blue points inner thin-disc stars. The red line in panel a) shows the selection criterion of outer thin-disc stars as defined in Eq. (12). The size of the points is age for panel a), [Fe/H] for panel b) and [Si/Fe] for panel c). The horizontal lines give the estimated error on the ages for young (blue, 1 Gyr) and old (orange, 1.5 Gyr) stars. (This figure is available in color in electronic form.)