To test these hypotheses , we examined heart rate and blood pressure before and after administration of placebo and 2 doses of caffeine ( 1 and 2 mg/kg ) in prepubertal ( 8 - to 9-year-olds ; n = 52 ) and postpubertal ( 15 - to 17-year-olds ; n = 49 ) boys ( n = 54 ) and girls ( n = 47 ) by using a double-blind , placebo-controlled , dose-response design .