Objective: To investigate the early life risk factors for low bone mineral density (BMD) in preschool aged children. Methods: Children aged 5 years from the Ewha Birth & Growth Cohort were followed from September 2010 to November 2011. The subjects included 146 who had their BMD measured using quantitative ultrasound densitometry. Those in the lowest quartile of BMD were classified as the low BMD group (n=35). The early life risk factors considered were birth outcomes, advanced maternal age (≥35 years), breast feeding, current anthropometric data, socioeconomic status, and physical activity. Results: The mean BMD was 68.1±13.4 (boys 67.7±12.1, girls 71.1±20.9, p>0.05). For early life risk factors, 17 subjects who were born to women of advanced maternal age belonged to the low BMD group, it was a significant difference when compared with another group (37.8% vs. 15.6%, p<0.01). In addition, the proportion of low BMD increased with the average time in sedentary activities, although not significantly (p-value for trend=0.06). No associations for other risk factors were found. In the multiple logistic analysis, advanced maternal age was independently associated with offspring low BMD (OR 3.7, 95% CI 1.6~8.9), even after adjusting for sex, sedentary activities, and household income. In addition, children who spent 1 hour or more a day in sedentary activity tended to be at increased risk of low BMD, but this was not significant. Conclusion: Advanced maternal age is negatively associated with the BMD of their offspring in early life. Further research is required to improve the health of the next generation, taking into consideration advancing maternal age.