Strings age due to two different and simultaneous processes. First and most obvious - the finger debris that gets ground into the windings of wound strings.

Secondly, strings vibrate most accurately when they're as close to perfectly round as possible. Nylon bass strings are wound with very soft metal on a core of bundled fine fibers of silk, nylon or other synthetics. As these strings get repetedly pressed into the frets, they quickly get little flattened areas which eventually ruin their ability to play in tune. At that point the only recourse is to change them.

One set of treble monofilament nylon strings usually will outlast 2 - 3 sets of basses.