Assuming you are asking for the average height of an eight year old boy, the answer would be 50.5 inches, or 128.3 cm.
This is based on data from the Centers for Disease Control and Prevention (CDC), which states that the average height of an 8-year-old boy in the United States is 50.5 inches, or 4 feet 2 inches.
The average height of an 8-year-old girl in the United States is 49.5 inches, or 4 feet 1 inch.
In general, boys will be a couple of inches taller than girls at this age. However, there is a wide range of normal heights for 8-year-olds, so some boys may be taller than others his age and some girls may be taller than other girls her age