Complex sentence: At the age of 21, he created Archie for MLJ's "Pep Comics" (December, 1941), and the success of the character led MLJ to assign Montana to do the first issue of "Archie" (November, 1942).
Split into simpler sentences: At the age of 21, he created Archie for MLJ's "Pep Comics" (December, 1941). The success of the character led MLJ to assign Montana to draw the first issue of "Archie" (November, 1942).