Age–metallicity distribution for the bulge GCs from the literature (black symbols) and for Terzan 6 (large red square). The literature data are mainly from Saracino et al. (2019, see their Figure 16), Oliveira et al. (2020, see their Figure 12), and Cohen et al. (2021), with the addition of a few recent age determinations for NGC 6440 (Pallanca et al. 2021), NGC 6256 (Cadelano et al. 2020b), and NGC6316 (Deras et al. 2023). The age and metallicity of the oldest stellar population in the two suspected bulge fossil fragments (namely, Terzan 5 and Liller 1; Ferraro et al. 2009, Ferraro et al. 2021) are plotted as black triangles. The gray vertical strip marks the weighted average and the 1σ uncertainty (12.6 ± 0.6 Gyr) of the entire sample.