Over their lifetimes, self-luminous brown dwarfs and giant planets cool and fade, giving rise to changes in their atmospheres. This cooling is itself mediated by the atmosphere (and interior), which is a key challenge in computing self-consistent evolutionary models. Empirically determining the influence that changing atmospheres have on the evolution of substellar objects requires measurements of their fundamental parameters of mass and/or age. In recent years there has been major progress in using high-precision mass benchmarks to measure the influence of the L/T transition on substellar cooling rates. The strongest tests of evolutionary models use binary brown dwarfs with individual dynamical mass measurements as "mini-clusters" to probe luminosity/temperature evolution. There is now ample evidence that cooling slows during the L/T transition, an effect that is not included in most evolutionary model grids. While theory has successfully predicted some aspects of luminosity evolution, there remains tension between some substellar cooling ages and other independent age-dating methods.