The equal division of goods is a long-existing social norm present in societies around the world. In order to ensure that the egalitarian norm is followed, people engage in costly enforcement of norm-violating behavior. Despite its importance, little is known about the emergence of this enforcement and how it develops over time. Therefore, we take the most commonly-used third-party punishment game where a third party is added to a dictator game, adapt it for children and run an experiment with 9-18 year-old children and adolescents. We show that already at 9-10 years of age, a small but non-negligible proportion of subjects are costly enforcing the egalitarian norm. We find that this behavior then strongly develops in the following years: The proportion of egalitarian norm enforcers increases, becoming the most common behavioral type with 11-12 years of age, and the punishers' behavior fully develops until 13-14 years of age. Following those developmental changes, the enforcing behavior remains stable until adulthood. We find that some norm enforcers do not only punish selfish, but also generous deviations from the egalitarian norm. Looking at the dictators' behavior, we observe that they increase their transfer in the direction of the egalitarian norm primarily in the same period as we observe developmental changes on the punishers' side.