A bit complicated
It depends on their age and how tall they are.
For a 13 year old who is at the 50th percentile for their height
(half the kids their age are taller and half are shorter), for both boys and girls, the 50th percentile for blood pressure is about 110/65. A more important question is what is a high blood pressure
. For a 13 year old at the 50th % for height, about 120/80 is above 90% for bp.