In Moses 6:31, Enoch describes himself as a “lad”:
And when Enoch had heard these words, he bowed himself to the earth, before the Lord, and spake before the Lord, saying: Why is it that I have found favor in thy sight, and am but a lad, and all the people hate me; for I am slow of speech; wherefore am I thy servant?
The Bible does not refer to Enoch as a “lad”, but the idea of Enoch as youthful does appear in the pseudepigraphal books of 2 and 3 Enoch (found at the end of the nineteenth century, well after the Book of Moses was published).
Where did Joseph Smith get this information on Enoch from?
See: