Like has been said, I wouldn't worry too much about age. I got mine 2nd hand about 12 months ago and only used it a handful of times. They are a superb lens and absolutely pin sharp.
If I was looking now, I'd be more interested in the condition of the body (don't worry about the rubbing where the lens hood attaches as they all have this), the focus rings rubbers should be clean and smooth, and make sure it's a sharp copy (although I've yet to come across anyone complaining about a soft copy).
Out of interest, how do you find the 600/4? I've been considering the 500/4 IS & 600/4 IS, but wondered how you manage with the non-IS?