Allright. Since you're tossing out the lore debate and want to focus on unchangable real life, lemmie pull something out. Remember when you said
Well, it does exist. Here's two links for you.
http://en.wikipedia.org/wiki/HeLa
http://en.wikipedia.org/wiki/Jurkat
Know what these are? These are immortal cell lines. That's right, I said immortal. They're cancer cells that will not stop growing or dividing, they will not grow old and die. And they're real- HeLa is commonly used in cell research and can be a contaminant in laboratories while Jurkat cells are used to study T cell Leukemia. Biological immortality is real. And if it's real, it can apply to dragons.
Dont like the Wikipedia links? Here ya go!
http://www.smithsonianmag.com/science-nature/Henrietta-Lacks-Immortal-Cells.html
http://www.copewithcytokines.de/cope.cgi?key=Jurkat