This study used a randomized online survey design to examine the potential influence of ageism on physical activity (PA) prescription for hypothetical patients with prediabetes of different chronological ages. Participants included 356 kinesiology students who were randomly assigned to one of four conditions. Each condition presented a clinical case study of a hypothetical patient with prediabetes referred to the participant as a qualified exercise professional for a PA intervention. Case studies were identical except for the reported patient's chronological age. Participants provided recommendations for PA support, frequency, duration, and intensity. Significant main effects were observed for PA duration, F(3, 352) = 8.62; p < .001, and intensity, F(3, 352) = 16.20; p < .001. The older patient was prescribed significantly lower PA duration and intensity compared to younger patients and no-age control. This study provides evidence for the presence and independent influence of ageism against older patients in PA prescription for prediabetes management.