The treatment in the actual text of the book is rather different, and goes into some delightful detail concerning Tarzan's self-study program, which begins at the age of 10 or so:

  "Among the other books were a primer, some child's readers, numerous picture books, and a great dictionary. All of these he examined, but the pictures caught his fancy most, though the strange little bugs which covered the pages where there were no pictures excited his wonder and deepest thought."

These are materials his parents has gathered to take to their Colonial posting for the express purpose of educating their infant son over the next several years; he just gets a rather late start on the program.

  "He did not accomplish it in a day, or in a week, or in a month, or in a year; but slowly, very slowly, he learned after he had grasped the possibilities which lay in those little bugs, so that by the time he was fifteen he knew the various combinations of letters which stood for every pictured figure in the little primer and in one or two of the picture books."

He takes several more years to piece together some of the mysteries of verbs and modifiers... I've always found it a fascinating thought experiment about language formation, given the era. In this context, I suppose the Hollywood presentation of Tarzan could be considered an application of the critical period hypothesis (had it existed at the time).