The word goes back to operative days.  The Freemasons of the middle ages were a select group; they were the highest class artisans of their time.  It required sound health, moral character, high intelligence, to be a good operative Freemason, permitted to work on the great Housed of God which were the Freemasons’ work.  They were proud of their abilities and of their reputation and strict in their rules.

To become a Freemason a young lad was required to serve a seven year apprenticeship before he might ask to be permitted to make and submit to his superiors his “Master’s Piece” and be admitted as a “Fellow of the Craft.”  Before he could serve his time he had to prove himself; therefore he served a period of time as an Apprentice.  If at the end of that period he had shown himself possessed of the necessary qualifications of industry, character, decency and probity, he was “entered” on the books of the Craft and became an Entered Apprentice.”

Originally an Apprentice was not a member of the Masonic Craft, even after being entered on the books of the lodge; not until he had passed his apprenticeship and been accepted as a Fellow was he a Craftsman.  This practice gradually gave way to the modern idea and after 1717, Apprentices initiated in lodge formed the build of the Craft.  Ritual teaches that the Apprentice is a symbol of youth, the Fellowcraft of manhood, and the Master of old age; probably this conception is derived from the fact that learners, beginners, are young, experts are men, and the wise and learned the elder group.

One Hundred One Questions About Freemasonry, The Masonic Service Association