*HEALTH WARNING* This reply contains amounts of word that may, in some cases, be considered wall-like. Those of a lazy or literarilly challenged (now thats irony!) disposition are advised to strongly consider the consequences of continuing.
Not quite sure which one is right, cos i can't remember the exact wording. However, Dr. Halsey says this in Fall of Reach
"All of the clones will die of natural causes [at young age]"
"All of the clones have [already] died of natural causes"
1st quote correct: Clones have died way before 2552 (when, as has been pointed out in this thread by MaxRealflugal, the op, John 117 is 42)
2nd quote correct: Clones are dead during the time of the novel Fall of Reach. This novel's plot precedes the halo CE events, which in turn, precede the events that will occur in ODST.
So sorry, but no cigar.
It would however be an excellent plot twist
However.... What if the UNSC took flash cloning to further steps, later on in the Halo universe's fictional timeline? What if a flash clone of John-117 was created in, say 2550, two years before the Halo games' time, being Buck? Is this possible?
Considering that in that universe it is possible to go to other planets in a matter of days/ hours, faster than the speed of light in another dimension, hmmm. Yes I think that cloning (which we have already accomplished with sheep etc.) should be possible in that universe, oh yeah, lets not forget reactive energy shielding to boot. I'm going to say that they probably have quite large advances in technology.
P.S. {In reference to using fiction from novels} Yes I know that Bungie did not write the book, so one could argue that they will not stick to this idea of the flash clones dieing at a young age as it is not their fiction. Problem is, I have read in some weekly updates , or heard in some podcasts, Bungie employees refering to the "Halo [fiction(?)] bible" and referencing pieces of the Halo universe fictional story line and objects/ characters. Including Dr. Halsey.
Don't ask me to find them as, and I'm sorry to say this, I have better things to do with about 48 possible hours, like revision *sad face*
Thank you and sorry about the invariably large volume of text, kudos if you made it this far, have some milk and enjoy a dilectable cookie, on me! =]