IMPORTANCE No national standardized guidelines exist to date for venous thromboembolism (VTE) prophylaxis after pediatric trauma. While the risk of VTE after trauma is generally lower for children than for adults, the precise age at which the risk of VTE increases is not clear. OBJECTIVE To identify the age at which the risk of VTE after trauma increases from the low rate seen in children toward the higher rate seen in adults. DESIGN, SETTING, AND PARTICIPANTS Multivariable logistic regression models were used to estimate the association between age and the odds of VTE when adjusting for other VTE risk factors. Participants included 402 329 patients 21 years or younger who were admitted following traumatic injury between January 1, 2008, and December 31, 2010, at US trauma centers participating in the National Trauma Data Bank. MAIN OUTCOMES AND MEASURES Diagnosis of VTE as a complication during hospital admission. RESULTS Venous thromboembolism was diagnosed in 1655 patients (0.4%). Those having VTE were more severely injured compared with those not having VTE and more frequently required critical care, blood transfusion, central line placement, mechanical ventilation, and surgery. The risk of VTE was low among younger patients, occurring in 0.1%of patients 12 years or younger, but increased to 0.3%in patients aged 13 to 15 years and to 0.8%in patients 16 years or older. These findings remained when adjusting for other factors, with patients aged 13 to 15 years (adjusted odds ratio, 1.96, 95%CI 1.53-2.52; P <.001) and patients aged 16 to 21 years (adjusted odds ratio, 3.77; 95%CI, 3.00-4.75; P <.001) having a significantly higher odds of being diagnosed as having VTE compared with patients aged 0 to 12 years. These findings were consistent across the level of injury severity and the type of trauma center. CONCLUSIONS AND RELEVANCE The risk of VTE varies considerably across patient age and increases most dramatically at age 16 years, after a smaller increase at age 13 years. These findings can be used to guide future research into the development of standardized guidelines for VTE prophylaxis after pediatric trauma.