Estimation of stellar population age and stellar mass based on the observed F125W − K_s colour. Top panel: stellar population synthesis spectra from Bruzual & Charlot (2003) redshifted to z = 3.29 for various stellar ages in the wavelength range of interest. The spectra are normalised to a K_s-band flux of unity. The VLT/HAWK-I K_s and WFC3/IR F125W filter curves are shown for comparison. Lower left panel: evolution of the observed F125W − K_s (roughly rest-frame U − V) colour with stellar ages up to 1 Gyr and three metallicities. The measured colours and their uncertainty bands for Jil1, Jil2, and Jil4 are overplotted as horizontal stripes, while the upper limit for Jil3 is highlighted by arrows. Lower right panel: logarithmic mass-to-light ratio (log(M_⋆[M_⊙]/L_V[L_⊙])) as a function of stellar age. The confidence intervals in age for the galaxies are shown as vertical stripes, as determined from the stellar population synthesis spectra. The upper limit on the age for Jil3 is again highlighted by arrows.