Where most of the series' writers (including Ian Fleming) tended to fudge the issue of James Bond's age, having him miss the passage of time just as much as if he were put into suspended animation or flying about the galaxy at near-light speed between missions, William Boyd made a point of dealing with it in his 2013 novel Solo. He began the novel, set in 1969, with Bond marking his forty-fifth birthday.
In a note attached to the book he claimed that this was the most logical conclusion from what Fleming wrote, but did not spell out his reasoning (at which I have not been able to guess). However, I think it best to go with Fleming's most explicit reference to Bond's age, which unsurprisingly turns up in the early chapters of Moonraker. In the course of explaining the more mundane aspects of Bond's life (what the hell does he do when not on missions?) in more detail than he ever did before or since, Fleming remarked that Bond was doubtful he would make it to "the statutory age of forty-five" at which double-os retired. Which made it "eight years to go before he was automatically taken off the 00 list and given a staff job at Headquarters."
Some simple math makes Bond thirty-seven in 1955, and thus born in 1918--a decade younger than his creator (born in 1908). That would make Bond thirty-five in Casino Royale, and about a year older in every subsequent adventure, after as well as before Moonraker.1
The six years between this estimate, and Boyd's, make a good deal of difference. After all, Boyd's younger 007 hardly seems likely to have been able to perform those assassinations that got him his double-o rating (and indeed, Boyd drops this aspect of his past, presenting his World War II service as being of a different kind).
It also makes much of what followed unlikely. The world-weary Bond we meet at the start of Casino Royale does not seem the twenty-nine that Boyd's math makes him. Or thirty-six as he is packed off to Shrublands. In fact, Bond often seems older than the age Fleming implies in Moonraker, as when he grumbled to himself about his cabbie and everything else he didn't much like about the world on his ride to the spa. But that seems a matter of the writer simply not being able to resist putting his thoughts into his protagonist's mouth more than anything else. And if one is generously inclined, it's simple enough to say that the life of a double-o is no drink from the fountain of youth.
1. This had Bond bumping up against that mandatory retirement age circa On Her Majesty's Secret Service. The detail seems the more relevant because Bond was thinking of handing in his resignation at the start of that tale, and because at the start of You Only Live Twice Bond seemed washed up anyway--but Fleming chose to simply ignore that aspect of the character.
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