With insufficient mass to sustain core hydrogen fusion, substellar objects
continue to cool and fade in brightness after birth. Those heavier than abut 13 Jupiter masses, called brown dwarfs, manage to ignite deuterium or lithium, thereby maintaining hydrostatic equilibrium for a short period of time. Those lighter than this do not undertake any nuclear reaction whatsoever in their lives and evolve like planets. So far a few thousand brown dwarfs and planetary-mass objects are known, almost exclusively in the solar neighborhood, i.e., they are already aged. Characterization of the youngest substellar objects by spectroscopy is hampered by their faintness and often confusion with field contamination. We describe our international collaboration to select substellar candidates in nearby star-forming regions 1 to 2 Myr old, the era when brown dwarfs are being formed or in their infancy. Our sample of substellar populations in star clusters of known ages and distances provides stringent constraints on theoretical modeling of
ultracool atmospheres, and of chromospheric/coronal activity. We also present how these least-massive members as the most vulnerable in stellar dynamics to get ejected, leading to eventual disintegration of star clusters.