While a blood pressure of 120/80 is normal for people over 18, for those under age 18, normal is based on a percentile rating for the teen's age, height and gender, according to KidsHealth. A blood pressure above the 90th percentile is out of the normal range.
A teen's blood pressure reading at the 90th percentile up to the 95th percentile is considered borderline or prehypertension, and above the 95th percentile is considered high. KidsHealth indicates that the normal blood pressure range for children is usually lower than normal for adults. Children with blood pressures above the normal range are three times more likely to suffer from hypertension as adults. Teens in the normal range should have blood pressure checked annually. Those near or above the 90th percentile should speak with their doctor concerning the frequency of blood pressure checks.
The National Heart, Lung and Blood Institute has a table that lists normal ranges for teenagers. For example, at age 13, the highest normal blood pressure for boys in the 95th percentile for height is less than 126 systolic over 79 diastolic. Girls the same age in the 95th percentile for height should have blood pressure no higher than 124 systolic over 79 diastolic.