Distribution of IllustrisTNG galaxies across the plane defined by the average specific star-formation measured over the last 10⁸ (sSFR₈) and 10⁹ yr (sSFR₉). Quenched galaxies are found below the solid red line, whereas the dotted line delimits the retired domain. Ageing systems are located above both regions. The grey-shaded area denotes the forbidden region of the parameter space.