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