I'm not so sure of the terms being offensive, but rather useful in quickly describing a set of more or less universal qualities said person has. If there are any truly derogatory qualities in that set, I'm not fully aware of them.Maybe a good follow-up (or two) would be what qualities are relayed by the distinction newbie/oldbie, to see if they are universal and/or offensive.
newbie is a term experienced players are supposed to use to condescend to newer players. Consequently, it's always been my assertion that no experienced player should stand to be called oldbie, insist on being called a realms vetran.
I don't consider newbie offensive. I've refered to myself that way both in the game and other settings. I don't consider oldbie offensive, but I don't think of myslef as one. To me as long as there are players who were veterans when I started active (Jarrod, Dave, Steve J, Carrie, Andy D) I'm not an oldbie yet. I've been playing 14 years this July and only started considering myself a Veteran (which I see as pre oldie) a couple years ago.
I don't think the are offensive terms at all. labeling someone as a newbie I think can be a good thing because others I see make an effort to help them learn thought fighting spell path suggestions or just to relay the experiences they have had. As for Oldbie...I don't know how other people feel about it but it's kind of like a right of passage. You have been in the game so long that you are an oldbie.