Menarche age is influenced by socioeconomic status, environmental factors, population density, and physical characteristics such as height and weight. In Mongolia, there has been few research related to socioeconomic factors with age menarche. Bayanzurkh district is in the southeastern part of Ulaanbaatar city, covering the largest area (1,244.1 km²), and has the highest population among districts, with 436,822 residents. Uvs province is situated in northwestern Mongolia, covering an area of approximately 69,585–69,600 km² with a population of 84,000.

The study was conducted from April to June 2025 using a cross-sectional design. Participants included individuals aged 12 and over attending secondary schools in the Bayanzurkh district and Uvs province. Body mass index (BMI) was assessed according to the definition provided by the World Health Organization. All statistical analyses were performed with version R-4.5.1 (RStudio-2025.05.1-513).

A total of 486 girls were included in the study. Out of 286 girls aged 8-18 in Bayanzurkh district, the average age at menarche was 11.84 years. In the study, 44 girls (15.4%) who participated from Bayanzurkh district were 14 years old, while in Uvs province, 12.0% of the participants were 8 years old. In Bayanzurkh district, 31.5% of the girls lived in households with four members, whereas in Uvs province, 37.5% of the girls lived in households with five members. Regarding the mothers' education, 51.0% had a higher education. In contrast, 29.0% of the mothers in Uvs province had only low education. Among 200 girls aged 8-18 in Uvs province, the average age at menarche was 12.97 years. The age at menarche was earlier in areas with higher population density compared to areas with lower population density, and this difference was statistically significant (p<0.0001). In girls aged 12-17, the BMI in Bayanzurkh district was higher compared to girls in Uvs province of the same age group. The age at menarche for girls aged 12-17 in Bayanzurkh district was 1.13 years earlier than in Uvs province. A higher BMI was associated with an earlier age at menarche in both Bayanzurkh district and Uvs province. This correlation was strong in the 14-17 age group, where higher BMI was correlated to earlier menarche.

The population density of the province or city where the girls lived had an influence on the average age at which they experienced menarche.