The social environment influences neurodevelopment. Investigations using rodents to study this phenomenon commonly isolate subjects, then assess neurobehavioral consequences while animals are still isolated. This approach precludes one from dissociating the effects of on-going versus prior isolation, hindering our complete understanding of the consequences of social experience during particular developmental periods. Here, we socially isolated adolescent mice from postnatal day (P)31 to P60, then re-housed them into social groups. We tested their ability to select actions based on expected outcomes using multiple reinforcer devaluation and instrumental contingency degradation techniques. Social isolation in adolescence (but not adulthood) weakened instrumental response updating, causing mice to defer to habit-like behaviors. Habit biases were associated with glucocorticoid insufficiency in adolescence, oligodendrocyte marker loss throughout cortico-striatal regions, and dendritic spine and synaptic marker excess in the adult orbitofrontal cortex (OFC). Artificial, chemogenetic stimulation of the ventrolateral OFC in typical, healthy mice recapitulated response biases following isolation, causing habit-like behaviors. Meanwhile, correcting dendritic architecture by inhibiting the cytoskeletal regulatory protein ROCK remedied instrumental response updating defects in socially isolated mice. Our findings suggest that adolescence is a critical period during which social experience optimizes one's ability to seek and attain goals later in life. Age-typical dendritic spine elimination appears to be an essential factor, and in its absence, organisms may defer to habit-based behaviors.Humans and rodents who experience early-life traumas or adversities appear to be prone to habit-based behaviors, often occurring at the expense of goal-oriented actions. Despite consistencies across species, how adversity, particularly during specific developmental periods, causes long-term behavioral biases remains unclear. Compounding this issue, many rodent investigations using social isolation to model adversity test mice or rats while they are isolated, making it difficult to dissociate the consequences of current versus developmental hardship. We reveal that mice with a history of social isolation during adolescence are biased toward habit-like behaviors, despite social reintegration in adulthood. Biases are linked with abnormalities in glucocorticoid tone and prefrontal cortical dendritic spine elimination during adolescence and were corrected by manipulating actin cytoskeletal regulatory factors.