Sensitivity of the SED of GCs to absolute age, estimated for the six photometric bands considered in this study. The SEDs are shown in the top panels, for a delayed SFH with τ = 0.1 Gyr and Kroupa IMF (Kroupa 2001) (see Sect. 2.2 for more details), along with the filter transmission functions used for studying the Sparkler. The left panels show the typical SED of a young population (age ∼1.5−4.5 Gyr), while the right panels present an older population (age ∼11.5−14.5 Gyr). All SEDs are normalised to the flux in the redder photometric band (the one with the smaller error). The middle panels show the difference in magnitude with respect to a reference age, considering 3 Gyr and 13 Gyr, respectively for the left and right panels. The bottom panels report the significance of the estimated differences normalised by the typical errors in the various bands for the Sparkler’s photometry, where the shaded regions show the 0.25, 0.5, and 1σ regions.