The purpose of this study was to examine whether loneliness mediates the relationship between social engagement and depressive symptoms and to determine how age moderates the mediation effect. Data for this study came from the survey with community-dwelling adults aged 18 and older in South Korea, from March to April 2017. The total of 1,017 respondents were drawn from three age groups (18-44, 45-64, or 65 and older). The mediating effect of loneliness was tested between each of three social engagement-related variables (family network, friend network, and perceived community support) and depressive symptoms. The results showed age differences in mediation: the effect was most pronounced in the relationship of family network with loneliness for the older group, whereas the size of friend network significantly predicted loneliness for younger adults. Both younger and older groups felt less lonely when they had a higher level of perceived community support; the middle age group remained uninfluenced by the mediation effects. Our findings confirm that loneliness is one of the mechanisms by which social engagement exerts its effect on depressive symptoms. As the Korean society embraces its growing proportions of older adults, the results of the study provide implications for adaptive strategies for changing social engagement need and mental health associated with ageing.