Adult neurogenesis in the dentate gyrus may contribute to hippocampus-dependent functions, yet little is known about when and how newborn neurons are functional because of limited information about the time course of their connectivity. By using retrovirus-mediated gene transduction, we followed the dendritic and axonal growth of adult-born neurons in the mouse dentate gyrus and identified distinct morphological stages that may indicate different levels of connectivity. Axonal projections of newborn neurons reach the CA3 area 10–11 d after viral infection, 5–6 d before the first spines are formed. Quantitative analyses show that the peak of spine growth occurs during the first 3–4 weeks, but further structural modifications of newborn neurons take place for months. Moreover, the morphological maturation is differentially affected by age and experience, as shown by comparisons between adult and postnatal brains and between housing conditions. Our study reveals the key morphological transitions of newborn granule neurons during their course of maturation.