well...I had a pair of lightweight bars which I sold recently. They were bought in the early 90s, were ridden for around 50 miles, then went into a store cupboard in a cool dark place for over 30 years. They were subject to no stress, no heat and no chemicals other than cool air. On that basis they will have continued to possess all the metallurgical and physical properties they possessed when they were originally produced. I had no qualms selling them. However, the bars where I had a massive shunt and were subjected to huge destructive force, and twisted in the stem, causing a massive gouge - are are certainly fit only for recycling.
Old, thin walled, unused = absolutely fine
Old, thin walled, well used and never subjected to forces above design spec thresholds = almost certainly fine
Old, thin walled, hopelessly abused = discard
the original Pace sub130s were VERY thin walled. Below 130 grams and not ti - amazing. But then Renthal really know what they are doing. I rode them in VERY demanding conditions in their time. Trusted them, looked after them, and they looked after me.