Age and skinfolds have been shown to influence running economy in children and adolescents. The purpose of this study was to determine the effect of age and skinfolds on the achievement of an aerobic steady-state running performance in children and adolescents at random running speeds. Subjects were 93 children, ages 9-12 years, and 86 adolescents, ages 13-18 years. Each subject performed a submaximal running test at a randomly selected running speed between 4 - 8 mph . Heart rate and oxygen uptake were measured throughout the duration of the test. A submaximal aerobic steady-state running performance for each subject required at least two of the following: RER less than 1.0, heart rate less than 90% age-predicted max heart rate, and RPE less than 18 (6-20 scale). Height and body weight, as well as triceps and medial calf skinfolds were measured for each subject. Logistic regression analysis revealed a logistic relationship between treadmill speed and the achievement of a steady-state running performance (chi square=7.9, p<.05). Sum of two-site skinfolds (chi square=18.0, p<.05) and age (chi square=11.2, p<.05) were also significant predictors in the logistic regression model. The derived model indicates that at a speed of 6 mph, a 15-year-old with a sum of two-site skinfolds of 20 mm, the probability of achieving an aerobic steady-state running perormance is 89.5%. For a 15-year-old at the same speed, but with a two-site skinfold sum of 40 mm, that probability drops to 67.0%. For a 10-year-old with a sum of two-site skinfolds of 20 mm and at the same speed, that probability drops to 64.5%. These data demonstrate that younger children and adolescents are less likely to achieve an aerobic steady-state running performance at any speed than older children and adolescents. Also, an increase in sum of two-site skinfolds in children or adolescents of any age reduces the likelihood of an aerobic steady-state running performance at any speed.Keyword(s): assessment, exercise/fitness, measurement/evaluation