In reply to Dark-Cloud:
I haven't been climbing for long enough to have retired any gear, but the massive loss of strength of slings with abrasion makes those the most likely bits of my rack to replace first. Some gear manufacturers are using 8mm dyneema on gear now - in my mind, the thinner the dyneema, the less wear it can take, so the earliest it will have to be reslung. Not sure if there's been any tests on that, mind.
Apart from that, the BMC website has a good video on common wear found on different bits of a rack. I wouldn't retire metalwork on the basis of age. From memory: Nuts - look for breaks or corrosion in the wires, especially at the 180 degree turn above the nut which is the most stressed bit. Karabiners - I wouldn't retire on age, but watch for gates sticking or not closing properly, sharp edges (or corrosion). Cams - bloody everything.
I think my attitude is to give the rack a once-over every 6 months or so, retire textiles on age, and metal on wear.