As the title says.
According to this, he began talking about Bilbo Baggins to his children around 1933. That puts him at 41.
The Hobbit was finished in 1936 and published the next year.
Accounts indicate, however, that he began writing parts of the Silmarillion during a recuperation period he underwent for … I can’t remember if it was disease, or injuries… during World War I.
I wish I still had the term paper I wrote on Tolkien somewhere…
It’s not going to be clear, because Tolkien spent decades working on fantasy, languages and myth. Where do you draw the line?
In 1910, when he was 18, Tolkien wrote a poem about a woodland scene, which included references to ‘fairy things’ and ‘Spirits of the wood’.
Does that count?
Around 1912 (now aged 20) Tolkien started to create the language which became Quenya (High Elven).
Does that count?
In 1914 Tolkien wrote the poem ‘The Voyage of Earendel’. He was 22.
This surely counts!
(All these references come from the excellent biography by Humphrey Carpenter.
Tolkien was born in Bloemfontaine, O.F.S., in 1892, and raised in Warwickshire. He began an interest in languages while at university (Oxford) and apparently started the invention-of-languages process then.
He started work on a mythos based on Britain while invalided out of the British Army after the Battle of the Somme in 1916, when he was 24. The Book of Lost Tales contains what has survived of that earliest mythos, which was largely incorporated, in modified form, in The Silmarillion. This was pretty much a history of the Elves from the First Age, conceived of as the retelling of a series of stories to a man who ends up shipwrecked on the island of the Elves. And the conclusion of the Elf-cycle of stories with the voyages of Earendil (who, by the way, was cadged from old legend, and in fact makes a guest appearance in Hamlet under a variant name), was all that the Tolkien mythos encompassed at that time.
Some years later, he and C.S. Lewis agreed to begin writing the sorts of stories they enjoyed. Tolkien’s effort was a story based in the metempsychosis of a father-and-son pair, tracing them back in time to an earlier life in Atlantis. Inevitably some of his language-interest influenced the story, and he brought in elements of his mythos. This gave rise to the idea that the mythos continued on after the First Age cycle of stories ended.
Tolkien was fond of making up stories for his children. The non-Middle Earth fiction from his pen was in large part drawn from those stories. The Hobbit was originally one such story, whose publication is already the stuff of legend – he having written it up, it was loaned for the amusement of a sick child, found its way to Stanley Unwin, whose son Raynor read it at age ten, and changed publishing history by recommending to his father that it be published. (It must be an interesting experience for a publisher, which Raynor became in succession to his father at the family firm, to know that the biggest decision of his professional career was the one he made at age 10.)
In The Hobbit Tolkien again borrowed small elements of his mythos – there are references to Gondor, the names of the swords and the eagles are borrowed from the mythos, etc. Elrond, a small child in the Elf-mythos cycle, becomes a supporting character as a wise old elf (he had not yet become a Peredhil in Tolkien’s conception). But The Hobbit was conceived as a standalone children’s story.
When, however, he was called on to write a sequel to The Hobbit, he began to write another hobbit-story, and more and more elements of the mythos began to play elements in the story. And his decision to tie the two together through Gollum’s ring, coupled with the significance of a ring in Teutonic mythology (which Tolkien knew extremely well), led to the evolution of a Third Age, following the Elf-mythos cycle of the First Age, on which he continued to work, and the Numenor concept which became the Second Age.
The Hobbit was apparently composed in the early 1930s; Raynor reviewed it in 1936. He began work on the sequel prior to WWII, but did not complete it until 1949, and it was not published until 1954-55.
Many even try to tie Tolkien’s earliest printed work, Goblin Feet to Middle Earth given its subject matter. It was published in 1915.
If you want to know about the earliest work Tolkien did on his mythology, you should read Tolkien and the Great War by John Garth, which discusses Tolkien’s experiences in World War I and how it affected his writing (which was already started).
Slight nitpick, but that was Gondolin, not Gondor.