Compulsory education systems usually specify a cutoff date regulating the precise age for entry into primary school. Existing literature from the USA and Europe has demonstrated that children born just before the cutoff date are disadvantaged in academic performance and has formalized this phenomenon as the “age position effect.” A recent study by Liu and Li (Sociol Stud 6:169–245, 2015) reported similar findings for China. Our study, however, challenges Liu and Li’s conclusion by examining the long-term effects of birth month on socioeconomic attainment and the compliance rate regarding the cutoff date. Using the 2005 1% Mini-census Survey and the 1992 National Sample Survey of the Living Situation of Chinese Children, we found a significant variation in socioeconomic achievement by birth month in adulthood in ways that differ from those reported by Liu and Li. In addition, we found compliance to be affected by both birth month and parental characteristics. In conclusion, our article proposes parental self-selection as an alternative explanation for the cutoff date effect.