In The Children of Húrin:
They often went to battle with the Men of Brethil against the Orcs, who now harried the northern borders of their land; for Húrin, though only seventeen years of age, was strong, and Huor the younger was already as tall as most full-grown men of that people.
In The Silmarillion, “Of the Ruin of Beleriand”:
. . . in that time; and they went both to that battle with the Orcs, even Huor, for he would not be restrained, though he was but thirteen years old.