We present a study of 15 new brown dwarfs belonging to the $∼7$ Myr old 25 Orionis group and Orion OB1a sub-association with spectral types between M6 and M9 and estimated masses between $∼0.07$M$_⊙$ and $∼0.01$ M$_⊙$. By comparing them through a Bayesian method with low mass stars ($0.8\lesssim$ M/M$_⊙\lesssim0.1$) from previous works in the 25 Orionis group, we found statistically significant differences in the number fraction of classical T Tauri stars, weak T Tauri stars, class II, evolved discs and purely photospheric emitters at both sides of the sub-stellar mass limit. Particularly we found a fraction of $3.9^{+2.4}_{-1.6}~\%$ low mass stars classified as CTTS and class II or evolved discs, against a fraction of $33.3^{+10.8}_{-9.8}~\%$ in the sub-stellar mass domain. Our results support the suggested scenario in which the dissipation of discs is less efficient for decreasing mass of the central object. ; Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society. 15 pages, 9 figures and 5 tables