This paper estimates peer effects on children's school entry age, using a dataset on 4,165 children from rural northwestern China (Gansu province). Instrumental-variable estimation, exploiting variations in (older) peers' home-to-school distance to identify the effect of their school entry age, reveals that a one-year increase in (older) peers' school entry age raises a child's school entry age by 0.43 years. This effect is much stronger than the effects of family-background factors such as parental education and family wealth, suggesting that the dominant driving force of delayed school enrollment lies outside of the family.