Third-party punishment is an important mechanism to enforce norm-following. However, the underlying process that explains the development of third-party punishment is understood poorly. Here we examine to what extent age-effects and contemporaneous experiences of receiving unfair offers influence third-party punishment. In two studies, a total of N = 280 5- to 9-year-olds participated in a computer-based task in which they received either fair or unfair offers from another peer. In the subsequent test phase, children could punish unfair offers as an unaffected third-party. We found that with age, children become increasingly systematic in their decisions to punish unfair allocations. However, there was no strong evidence that an immediate experience of (un)fairness influenced children’s punishment. Together, our results suggest that children develop a sophisticated application of fairness norms with age that is not easily swayed by their immediate experience of being treated unfairly.