Should network cables be replaced?

Jyms writes: As technology changes, so hubs routers and switches are upgraded, but does the cabling need replacing, and if so, how often? Coax gave way to CAT 5 and CAT 5e replaced that. If you are running a 100Mbit/s network on old CAT 5, can that affect performance? Do CAT 5(e) cables get old?