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Infancy is a highly vulnerable period in life. Most mammals are born unable to eat, locomote, or keep warm independently. Infants do, however, show several behaviors that are unique to their developmental stage and essential for survival and growth, such as frequent sleep or parent-directed vocalizations. How the infant brain controls behavior remains unclear. To address this, we focused on the preoptic area (POA) of the hypothalamus, a brain region in which genetically defined cell types have recently been linked to specific homeostatic functions and social behaviors in adult animals. We molecularly profiled POA cell types in mice using single-nucleus RNA-sequencing at nine ages, encompassing developmental transitions such as birth, weaning, and puberty (in press at Nature). This revealed (a) diversification of ~150 POA cell types soon after neurogenesis; (b) onset of mature neuronal gene expression patterns along cell-type-specific timelines, coincident with behavioral changes; and (c) a major role for early life experience in POA cell type maturation. This dataset will serve as a foundation to identify the molecular, cellular, and circuit mechanisms underlying how the infant brain controls behavior.