The personality of children and adolescents is related to many aspects of individuals’ later developments, including physical and psychological health, educational and occupational achievements, as well as social relationships. It can provide the basis and reference for the work of their personality development and cultivation to clarify the age differences in children and adolescents’ personality. However, the existing developmentally oriented research has not reached a consensus on the age differences in personality in western children and adolescents, and the debate on stability and change of personality development has always being existing. Based on the research about the Big Five personality development of western children and adolescents in the last three decades, this article distinguishes two main forms of personality development, and summarizes the age differences in the Big Five personality from childhood to adolescence, including the development of the Big Five personality structure, as well as the mean-level age differences in the Big Five personality domains. The Big Five, a hierarchical structure, is not only suitable for organizing the personality traits of children and adolescents in much the same way as it does to adults, but also relatively stable across childhood and adolescence. Although the number and the representativeness of low-order traits in some high-order domains may change, the five high-order domains remain stable from childhood to adolescence. The mean levels in the Big Five personality domains across childhood and adolescence vary with age, and the changes do not simply represent a monotonic trend toward greater and greater psychosocial adjustment. Extraversion gradually declines from childhood to adolescence. Agreeableness and Conscientiousness show the similar developmental trajectories: they both increase from early childhood to late childhood, decline during early adolescence, and increase again after entering middle adolescence. The development of Neuroticism during this period shows interaction effects of gender and age. From early childhood to middle childhood, the mean levels of Neuroticism for boys and girls rise slightly. From late childhood to middle adolescence, Neuroticism increases for girls, while declines for boys. After entering late adolescence, Neuroticism for boys and girls declines again. The mean-level change in Openness may be as follows: it increases gradually from early childhood to middle childhood, then declines from late childhood to middle adolescence, and at last increases gradually after the beginning of late adolescence. The mean-level age differences in the Big Five personality domains are associated with social culture, individuals’ life experiences at different stages and the corresponding biosocial changes and challenges which children and adolescents have to face. In the end, the paper proposes that future research should focus on cross-culture comparative studies about the personality development for children and adolescents in different cultures, and pay attention to the developmental characteristics of the low-order traits in the Big Five personality domains, and should also emphasize the developmental trajectories of personality extremity from childhood to adolescence. According to the current network era, the paper puts forward that future research should make full use of the computer-based personality judgments. To a certain extent, this article has integrated previous related studies and solved the long-standing debate on the stability and change of the personality development in children and adolescents.