Selected gravity-sensitive spectral indices from the literature versus the NIR SpT derived in Sect. 4.1. Our sample is divided into the different age classes presented in Sect. 2.1 where the classes <30 Myr (USCO, TWA, BPMG, and 32OR), 30–100 Myr (ARG, COL, and THA), 100–300 Myr (ABDMG, CAS, CARN, and PLE), and 600 Myr (PRA) are shown as green stars, black pentagons, red diamonds, and white squares, respectively. The complementary dataset is divided into young, mid-gravity, and field objects represented as orange, blue, and gray dots, respectively.