Spectral indices used for the classification and age estimation computed on SINFONI spectra of dwarfs classified in the optical (orange triangles) and in the NIR (filled orange triangles), on medium-resolution spectra of young dwarfs classified in the optical (red filled squares; the references associated to the spectra are given in Sect. 3.1.2), and on field dwarfs spectra from the IRTF library (filled blue dots; Cushing et al. 2005; Rayner et al. 2009). We also overlaid the indices’ values for the young late dwarfs found and classified by Lodieu et al. (2008) (violet stars) and our proposed alternative classification using the method described in Sect. 3.1.2 (filled green inverted triangles; see Appendix B).