In reply to John Clinch (Ampthill):
From the article:
"In the CE test, components failed on several harnesses that did not show previous visible damage. Even though a harness component may look acceptable, it may be the weakest link in the rig."
"However, the older a harness, the greater the potential for degradation of structural components such as buckles, threading and webbing."
"As expected, the more days a harness had been used, the lower the average breaking strength of both belay loop and leg loop tie-in point."
"Harnesses that are used more outdoors tend to have lower belay- and leg-loop strength than indoor harnesses, likely due to the abrasive nature of the outdoor climbing environment, exposure to UV and dirt."
In other words, it doesn't matter if thick webbing is stronger than thinner webbing, or if it doesn't show signs of wear -- a harness you've been using regularly for 18 years, especially if a lot of that's been outdoors, is potentially going to be very seriously weakened.