Osteoporosis is a major public health problem worldwide. Lower peak bone mineral density (BMD) in youth may be the single most important factor leading to the development of osteoporosis in the elderly. Using cross-sectional data from the National Health and Nutrition Examination Survey (NHANES) 2005–2014, we included 18,713 individuals with complete and valid data on femoral neck, total hip and lumbar spine BMD. Generalized additive models were used to estimate the age at attainment of peak BMD and 95% confidence intervals (95%Cls);​ model covariates were sex, race, body mass index (BMI) and we also examine factors potentially affecting age at attainment of peak BMD. This study clearly stated that age at attainment of peak femoral neck, total hip and lumbar spine BMD were 20.5 years, 21.2 years and 23.6 years in males, and 18.7 years, 19.0 years and 20.1 years in females, respectively and age at attainment of peak BMD varied by skeletal sites and sex. The study also found that females achieved peak femoral neck, total hip and lumbar spine BMD earlier than males (all P < 0.001);​ race and BMI were not associated with the age at attainment of peak BMD (all P > 0.05). These results suggested that improving bone health among individuals before 20 years old may be useful for reducing future risk of osteoporosis and osteoporotic fractures.