Cognitive development in early childhood and cognitive preservation in older adulthood are critical for leading healthy life. Social engagement can significantly affect cognition, but their relationships are unclear. The purpose of this review was to synthesize current findings on the relationship between social engagement and cognition in early childhood and older adulthood. PubMed, Web of Science, Ovid Medline, and PsycINFO were searched for studies published in 1995-2015 for a comprehensive review. Included in this review were 42 articles written in English, published in peer-reviewed journals with participants’ age being 2 to 6 or ≥65 years, and measurement of social engagement and cognition. Overall, greater social engagement was associated with higher levels of cognition across the life span, association of which seemed more prominent in populations at risk of cognitive impairment. Additional research is needed to elucidate biobehavioral mechanisms underlying these relationships and to test the efficacy of new interventions.