Well in 1721 Benjamin Franklin was 15 years old, and apprenticing for his brother (James Franklin) at his printing & publishing enterprise in Boston entitled The New-England Courant. You can read several of the publications here, if you're interested.

During his time working for his brother, he was submitting letters for publication under the alias 'Mrs. Silence Dogood." These letters were rather popular around town, and also considered rather inflammatory for their time. Benjamin's brother was actually jailed for publishing material critical of the governor at the time, who was then William Dummer, the Governor of the Province of Massachusetts Bay (a geographical area that encompassed some of present day Canada).

Now, it is important to remember that this was a time period that preceded most of the revolutionary rhetoric that sparked the American Revolution, and that the Governors were delegates of the British government.

For Benjamin Franklin to compose critical writing, and his brother to facilitate the publication of literary criticism, was very much a shocking, unacceptable, and illegal behavior that predated the formal promulgation of the 'Freedom of Speech' in North America by more than five decades.

So, on its own it was very independent thinking from young Franklin in 1721, who went on to publish another article/editorial critical of the government following his brother's brief incarceration in 1722 for the article mentioned above about then Governor Dummer.

The term "Free-thinker" emerged in England in the late 17th century. A man named Anthony Collins published a very popular book in 1713/1714 entitled Discourse of Free Thinking.

It was at this time that the concept of "free-thought" or rather of a "free-thinker" found its place in the popular public lexicon. I think it is easiest to describe this concept, as it was, as really mostly just a secular viewpoint. "Free-thought" mostly was a label for the critical thinking, rhetoric, exposition, or discussion about the Church of England, and the ineffective relationship between the Christian faith and governance by the monarchy.

This above piece, and the discourse that preceded and followed this book's publication, is the answer to your question. There were early parameters on what was considered "Free-thought" and it largely was an academic or philosophical maxim critical of religion's role in a government or a monarchy. This is quite a simplification of the maxim, I will say, but that is certainly the foundation of the free-thought scholastic movement in the early 18th century.

So your in-text citation to The Harvard Classics volume, which was actually edited and compiled by Charles Willian Eliot (who was the president of Harvard following the American Civil War) can also help answer this question.

For the author of the the biography to say that this recognition of Benjamin Franklin as a "free-thinker" occurred in 1721 is quite telling actually because this is when he was first writing and publishing work critical of a British delegate and of the colonial governance by the very church-handed British government.

So I think it is quite fair to say, based on the chronicles on emergence of the free-thinking movement in the early 18th century, that his publication of writing critical of the British government earned him a seat in the "Free-thinker" camp, as it was in 1721, when he started writing for the the Courant.

Free-thinking is an egis now days under which most scholastic thought could be argued to fall, but at the time it had a particular meaning, which you've asked about, that really was meant to characterize thought and discussion critical of Western European theocracy.

Great question!

I think it can be assumed that his use of the phrase "free-thinker" comes from his rejection of Puritanical ideology at a young age. Franklin was influenced by the 'Great Awakening' movement of the early 18th century where many Christians left their rigid denominations for more liberal interpretations. Most who were influenced by the Great Awakening did not reject God, but rather organized Christianity. Franklin falls into this category as he gives plenty of evidence throughout his life that he believes in God. He was one of only three men at the Constitutional Convention that felt prayer was needed before they began (Franklin, Madison, Jefferson).

Here's one quote by Franklin that gives insight: "I believe in one God, the creator of the universe; that he governs it by his Providence; that he ought to be worshiped; that the most acceptable service we can render to him is doing good to his other children..."

Another: “Lighthouses are more helpful than churches”

This one is very interesting by Franklin: "As to Jesus of Nazareth, my Opinion of whom you particularly desire, I think the System of Morals and his Religion, as he left them to us, is the best the World ever saw, or is likely to see; but I apprehend it has received various corrupting changes, and I have, with most of the present dissenters in England, some doubts as to his divinity."

In short, Benjamin Franklin, and just about every founding father, believed in the general teachings of Christ, believed in God as the "author of salvation", "law of nature", and that he should be worshipped. But, they generally rejected the entrapment of organized Christianity. He was a free-thinker in the sense that he believed the Christianity of his day was a hindrance to rationale, and exploring a greater truth.