**Background:** Most research on the health effects of school-entry age comes from high-income countries (HICs), with little evidence from low- and middle-income countries (LMICs). This scoping review aimed to map the empirical evidence on the effect of school-entry age on health in LMICs and identify future research directions. The authors hypothesized that school-entry age may affect health through pathways such as total years of schooling, access to school-based programs, peer effects, and maturity differences.
**Methods:** The review systematically searched health sciences, education, economics, psychology, and general sciences databases (PubMed, EconLit, ERIC, EBSCO, Google Scholar, African Education Research Database) between August and September 2022. The exposure was relative age for grade (young-for-grade or old-for-grade). Inclusion criteria were quantitative or qualitative studies reporting health outcomes in LMICs. Exclusion criteria included studies not related to school-entry age, not reporting health outcomes, from non-LMICs, or grey literature. Study characteristics were extracted, and findings were categorized into health domains that emerged a posteriori: neurodevelopment and mental health, sexual and reproductive health, non-communicable diseases, and nutrition.
**Key Results:** The search yielded 4919 documents, with 8 studies meeting inclusion criteria (4 from databases, 4 from citation searching). All studies were from middle-income countries (Brazil, Mexico, Vietnam, Türkiye) and published between 2017 and 2022. Three studies used quasi-experimental designs (Brazil, Mexico, Vietnam), and five were observational (Türkiye). No studies from low-income countries were identified. Key findings included:
- **Neurodevelopment and mental health:** Young-for-grade children had higher ADHD diagnosis rates. In Türkiye, prevalence of ADHD among early starters (<72 months) was 15.9% vs. 6.4% among late starters (78–83 months). In Brazil, risk ratios for ADHD comparing youngest vs. oldest children ranged from 1.11 to 2.02.
- **Sexual and reproductive health:** In Vietnam, starting school late reduced teenage motherhood by more than one third among women aged 15–19. In Mexico, young-for-grade women had earlier sexual debut, higher rates of adolescent pregnancy, cohabitation, and marriage, and among pregnant women, fewer prenatal care visits and more pregnancy complications (e.g., excessive bleeding).
- **Non-communicable diseases and nutrition:** In Mexico, young-for-grade women who had ever been pregnant were 9 percentage points more likely to report drinking alcohol, 8 percentage points more likely to smoke, and 3 percentage points more likely to use drugs before first pregnancy. In Brazil, being young-for-grade reduced stunting probability by 1.5 percentage points but increased overweight risk.
**Clinical Implications:** The findings suggest that school-entry age policies may have significant health consequences in LMICs, particularly for neurodevelopment, sexual and reproductive health, and risky behaviors. However, the evidence is limited to middle-income countries, and no studies assessed infectious diseases, most non-communicable diseases, or violence. The authors call for more quasi-experimental studies, inclusion of low-income countries, use of biomarker data, and qualitative research to understand mechanisms and inform interventions (e.g., tailored tutoring, delayed school entry) to offset potential disadvantages from school-entry cut-off dates.