Median [Fe/H] abundance, as a function of age, for our RR Lyrae population models. Different colours refer to different metallicity distribution functions. Solid lines refer to models with the Marconi et al. (2015) instability strip prescription and dashed lines refer to models with the Smolec & Moskalik (2008) instability strip prescription. The shaded area indicates the uncertainties related to the red giant branch mass loss. The solid black line shows the median metallicity measured for our sample of bulge RR Lyrae stars. The arrow marks the age of the Universe, according to the cosmological parameters of Planck Collaboration VI (2020). (a) Comparison with the spectroscopic metallicities. (b) Comparison with the Smolec (2005) photometric metallicities.