The survivorship of contemporary total hip arthroplasty has improved substantially as a result of the success of highly cross-linked polyethylene. Nevertheless, there is limited information on its performance in extremely young patients. The purpose of this study is to evaluate the (1) clinical and radiographic outcomes, (2) polyethylene wear rates, and (3) mid-term survivorship of total hip arthroplasty in patients 21 years or younger using highly cross-linked polyethylene. After Institutional Review Board approval, the authors identified 45 patients aged 21 years or younger (56 hips) who underwent primary total hip arthroplasty with highly cross-linked polyethylene at their institution by the senior author between 2000 and 2009. Patients were followed for a minimum of 2 years, or until revision. At a mean follow-up of 57 months, modified Harris Hip Scores, Western Ontario McMaster Universities Arthritis Index scores, Short Form-12 physical function scores, and University of California, Los Angeles activity scores improved significantly in all patients. There was no radiographic evidence of osteolysis in any patient. The mean linear polyethylene wear rate was 0.02 mm per year. Survivorship was 98% with one revision for acetabular loosening at 71 months. Total hip arthroplasty in patients 21 years or younger with highly cross-linked polyethylene had excellent clinical and radiographic outcomes at mid-term follow-up. Longer-term data are needed in this patient population to confirm continued survivorship in the extremely young. The level of evidence used for this study was level III.