In many mammalian species, animals form subunits within larger groups that are often associated with kinship and/or age proximity. Kinship mediates fission/fusion social dynamics of giraffe herds, but the role of age proximity has been unexamined. Here, we analyze 34 years of data from a population of Thornicroft's giraffe, Giraffa camelopardalis thornicroftii, living in Zambia in order to assess the extent to which age proximity influences herd composition. We show for the first time that calves born into the same cohort have stronger social associations than calves born into different age cohorts, and that the strength of their association is independent of the strength of maternal associations. Duration of time co-resident in the population did not influence the strength of social associations. Mothers and adult daughters have significantly stronger social associations than do unrelated adult females. We suggest that giraffe have evolved mechanisms for fostering the formation of social associations with similar aged non-kin. Giraffes live in a complex society incorporating both kinship and age proximity as factors modulating the formation of social associations that underlie the fission/fusion dynamics of their flexible herd structure.