Do adult-born granule cells (GCs) in the olfactory bulb differ functionally from those born during brain development? In this study, the authors assessed synaptic transmission in mature GCs of different ages. They found that presynaptic GABAB receptors suppressed GABA release probability in postnatally born GCs but not in adult-born GCs. Possibly related to this result, GABAB receptors had different localizations in postnatally born versus adult-born GCs. These findings suggest that adult-born GCs are functionally distinct from GCs born during development.