The adult mammalian olfactory bulb (OB) is continuously supplied with adult-born neurons. While some new neurons die shortly after arrival into the OB, others persist throughout the life of the animal. Here we followed the long-term morphological changes in adult-born periglomerular neurons and granule cells from the mouse OB well after they mature. We present a dataset of dendritic morphology and synaptic distributions from >100 adult-born neurons as imaged in vivo and reconstructed in 3D. The dataset currently includes a substantial range of neuronal ages (0.5–11 months old). Using this dataset, we show that the morphological steady-state which adult-born periglomerular neurons reach soon after maturation is not maintained in older neurons. Rather, total dendritic length decreases after 6 months of age. We find that this morphological decrease in “old” periglomerular neurons is regulated by the age of the animal, and is independent of neuronal age. This suggests that morphological development of adult-born neurons is regulated extrinsically. Our dendritic morphology dataset of 3D reconstructions is made available to the scientific community so it may serve as a useful resource for comparative morphological studies of the OB, and in particular of adult neurogenesis. J. Comp. Neurol. 519:2212–2224, 2011. © 2011 Wiley-Liss, Inc.