Self heating during long-term DC-aging is found to be responsible for the degradation of the electrical and optical characteristics of blue p-GaN/p-AlGaN/InGaN/n+-GaN/SiC LEDs. Electroluminescence and Cathodoluminescence studies reveal an additional large optical band, not observed in unstressed devices, on the p-type side of the LEDs. Deconvolution procedures shows the band, peaked at about 3.1 eV, is due to three main emissions at 2.93, 3.08 and 3.23 eV. Deep Level Transient Spectroscopy reveals the presence of four traps for majority carriers in p-type GaN. A comparison with the optical spectra suggests three of them (at 0.12, 0.22 and about 0.5 eV) are responsible for the additional emissions after stress, while a further trap at 1.21 eV is considered non-radiative in nature. A thermally activated mechanism inducing the dissociation of native Mg-H complexes and the subsequent formation of metastable Mg-H2 complexes is considered responsible for the LED degradation. The hypothesis is supported by high power electron beam irradiation during Cathodoluminescence studies which induced a complete disappearance of the band at 3.1 eV as a consequence of the dissociation under energetic electron beam of the metastable Mg-H2 complexes.