Well, yeah, I mean, I don’t think any of us have an epic elf character who is like “I’m 20 and have done ALL THIS AWESOME SHIT!“ or anything ;^^ Out of curiosity, do DK’s age past the point of when they were turned? Physically-speaking…

No. DKs remain the same after they are turned. It is by scourge magic itself that they retain their look and their body retains its unrotting glory. Honestly, I’d ask Tev or someone else. I’m not 100% sure on DKs. I don’t play one. 

Umm. Ask Sindre/Tev or Deance. They would definitely be able to tell you more.

Not normally, in the way the living do. Death Knights are immortal, Guide to the Side Effects of Reanimation mentions that undead should watch their rotting, however, Death Knights are known for their regenerative powers.