Family and intergenerational relationships are becoming increasingly important as sources of support and care for the elderly population in rapidly ageing Asian societies. However, this has also raised concerns over reinforcement of cultural preference for sons as a source of old-age security. This paper therefore revisits the question-what determines happiness in old age-by investigating the role of adult children's gender in the context of Thailand, an ageing Asian country with no legacy of sex-preference in fertility. We employ nationally representative data to examine the association between old-age happiness and presence of a co-residing child. Compared to living alone, living with at least one child is found to positively associate with older persons' happiness. However, this result is specific to daughters. Moreover, compared to older men, women systematically benefit from a "daughter effect". Co-residing daughters with university education and those who maintain a good relationship with their parents help explain the positive happiness effect on older persons. Co-residing daughters are also positively linked to reduced loneliness; improved self-rated health; and improved economic conditions of older parents. Our findings suggest that policies that increase human capital of the girl child and enhance family solidarity are likely to have long term intergenerational wellbeing benefits.