How old is old? I know there's forum members here that just don't change their strings (and some of those basses are the better part of a decade old, IIRC). There was another member who asked if Sheldon would start vacuum packing his strings to keep their freshness. There's people on both sides of the spectrum, but as far as I know, what "kills" strings is dead skin and oil from your fingers, not actual "age". This is why wiping strings down after playing and soaking old sets in alcohol helps preserve strings.
(Also, how old is the steel before it's processed into wire? How old is the wire before it's processed into strings? How long do the strings sit in a warehouse before hitting store shelves? Who knows how old strings are when they finally go on our basses?)
In short, although I've heard otherwise, I don't think strings sitting in stores for a few years will dampen that bright tone new strings have, nor make them brittle and prone to breakage. Buy with confidence.
If I am wrong, please someone correct me.