Early postnatal development encounters milk as a key environmental variable and yet the sole nutrient source. One evolutionary conserved constituent of milk is sialic acid, which is generally displayed on glycoconjugates and free glycans. During early postnatal development, high sialic acid need was proposed to be unmet by the endogenous sialic acid synthetic capacity. Hence, milk sialic acid was proposed to serve as a conditional nutrient for the newborn. In the elderly, at the other end of ontogeny, decreased sialylation in the brain, saliva, and immune system is observed. Analogous to the neonatal situation, the endogenous synthetic capacity may be unable to keep up with the need in this age group. The data discussed here propose a functional dietary role of sialic acid as a building block for sialylation and beyond.