Xelloss is 1000/1024 years old.
The Kouma War was about a millenium ago. Xelloss was around for the Kouma War, but that doesn't mean that was when he was created. So how old is he? Ask him. He'll probably give you an answer...

Xelloss is Zellas Metallium's priest and general.
Xelloss is Greater Beast's priest, not general. Zellas did not see the need to create a general.

Xelloss' full name is Xelloss Metallium.
Xelloss' full name is Xelloss.

Xelloss was once human.
This rumor, if not started by, was at least spread by inverse.org. (inverse.org has since deleted this misinformation from their profile of Xelloss.) The probability that Xelloss was once human is about the same as the probability that Lina can sit still for five minutes. Mazoku have little more respect for humans than they have for a cockroach. The idea of transforming a human into her priest would undoubtedly disgust Zellas...or make her laugh uncontrollably. Turning a human into a Mazoku would very probably not even work. There are human/Mazoku hybrids in the Dynast arc of the novels, but they are lowly creatures.

Filia is the last of her species.
Filia is the last of the Golden Dragons that worshipped the Fire Dragon King. There are plenty of other Golden Dragons worshipping the other three Dragon Kings still around.

"That's the Dragon I love."
-_-; "That is the Filia-san I know."