Hello everyone. :) Long time reader but first time writing something here myself.
I am pretty convinced that Dimitri Yuriev has been living since the Lost Jerusalem era, but not everyone seems to agree with my conclusions, so please let me elaborate. Some of you, who have played Xenoblade's latest expansion "Future Redeemed" might see where this is going, but my conclusion works with or without any Xenoblade references. So I will first start with the evidence that comes from Xenosaga games alone.
I know I'm someone who jumps to conclusions very fast, but in this case I am very certain that my logic is coherent. So if you disagree, please let us have a calm discussion here.
Until now we only knew that Dimitri Yuriev is at least hrundreds of years old, potentially older, because of his appearance in Pied Piper. As this already predates the U.R.T.V. project, we know that his body-hopping ability has nothing to do with U.R.T.V.s. In Xenosaga episode III, Sellers describes how Yuriev got his ability (as well as his driving fear) from contact with U-DO. We already know this. What is interesting is that Sellers says that this happened during the "early days of the U.M.N." And I admit that this is the thinnest part of evidence for my claim. But he wouldn't say "early days" if he actually meant thousands of years after the U.M.N. was created or established. So I am convinced that Yuriev is more or less as old als the U.M.N. itself. (minus a couple years or decades, or even a hrundred years, this would all count as "early days" considering how long ago this is from Seller's perspective)
So if we can assume that Dimitri Yuriev's age is similar to the U.M.N.'s, how old is the U.M.N.? And I'm not speaking of the subconcious domain itself. When Seller's talks about the "U.M.N.", he means the system administrated by Vector. We get the clearest evidence of the U.M.N.'s age with Grimoire Verum. As we know from A Missing Year, Grimoire Verum was a person from the Lost Jerusalem era, who researched Lemegeton and cuased the fist phase-shift phenomenon on his own daughter, Nephilim. Within his life-span he found a way to transfer his mind into the U.M.N. where he lived for thousands of years until the time between Xenosaga II and III in a Missing Year, long after his real body already decayed back in the Lost Jerusalem era. This clearly means that the U.M.N. was established by Vector during the Lost Jerusalem era.
And finally there is the new information we got from Xenoblade. I can't find a way to spoiler tag this here, so who knows, knows what I'm referring to. I know that everyone in the Xeno-communities, be it Gears, Saga or Blade resists from canonizing information from the other two franchises into their own franchise, because that might open a whole different can of worms. But Takahashi going out of his way, reaching out to Bandai Namco to have "Xenosaga" in the legal information of Xenoblade 3 in the game's home-menu and adding these elements to the game clearly shows his wish to canonize Xenosaga into the lore of Xenoblade. And I personally have the opinion that if the author wants something to be canon, it is canon. And most of all, it fits so well. The story of the Xenoblade games was written soon after or around Xenosaga's. I even think that early ideas already came as soon as the concept of a "Lost Jerusalem" first came up in Perfect Works. And when Dimitri Yuriev's role was written further in episode III, I am convinced that he was always intended as a key figure for what happens in Xenoblade, which was first hinted at in Xenoblade 2 and now even confirmed in Future Redeemed.
But even without that last paragraph, I have the opinion that the information inside Xenosaga episode III and A Missing Year alone is enough to prove that Dimitri Yuriev was born during the Lost Jerusalem era.