As Tarzan grew he made more rapid strides, so that by the time he was ten years old he was an excellent climber, and on the ground could do many wonderful things which were beyond the powers of his little brothers and sisters.
In many ways did he differ from them, and they often marveled at his superior cunning, but in strength and size he was deficient; for at ten the great anthropoids were fully grown, some of them towering over six feet in height, while little Tarzan was still but a half-grown boy.

Ch. 5 : The White Ape - Tarzan of the Apes (1912)