The 767 is still being built. All Nippon, Japan Air Lines and LAN Chile have taken delivery of new ones in 2007.
As with all aircraft types it is entirely dependent on how well you keep them up. I remember that BA's final One-Elevens in 1992, after 25 years of service, were still in immaculate condition inside and out when they only had weeks left to go. A few years after this I went with Virgin Atlantic on one of their 747-400s, probably about 4 years old, and couldn't believe how tatty the cabin fittings had become on it.
Age has nothing to do with it.
I get the feeling that the original poster has been looking at the BA fleet. Interestingly it divides into two sub-fleets, European and Intercontinental, with different seating configurations. The Intercontinental ones were refitted ("dusked") in recent times and look fine. The European ones have not been touched for a good while, and show it.