In the adult mammalian brain, there are dormant neuronal precursors located in various cortical and subcortical areas. Unlike progenitors of the canonical neurogenic niches, dormant precursors emerge prenatally and maintain a state of post-mitotic quiescence until they mature in response to yet unknown stimuli. Several reports indicated that the maturation of dormant precursors is rather slow and staggered across adulthood. Here, we directly compared the maturation rate of cortical dormant precursors to that of age-matched adult-born hippocampal neurons, derived from adult neurogenesis. To this end, in a transgenic mouse (DCX-CreERT2 /flox-EGFP), we labeled both neuronal precursor types at three months of age and tracked their maturation across adulthood, until nine months of age. The waning of immaturity markers (e.g., DCX) and the onset of maturity markers (e.g., NeuN) were abrupt in the hippocampal neurons, implying a quick maturation within weeks after labeling. Conversely, prolonged retention of immaturity markers and slow onset of maturity markers in cortical neurons suggested a much slower maturation of the latter, lasting for months after labeling. However, despite marker expression implying stark differences between the maturation rates of the two cell types, a neuroanatomical analysis revealed much closer maturation rates than anticipated. Accordingly, the progressive increase of dendrite length, surface area, and branching points of hippocampal and cortical adult-born neurons were comparable. Consequently, the existence of adult-born neuron networks of similar dendritic sizes and growth rates indicates that the maturation of both cortex and hippocampus is comparably slow, lasting for most of adulthood.