airframe fatigue depends entirely on flightcycles, not on airframe hours. I have worked on Fokker 100 with 49000hours/41000cycles but also on B747 with 101000 hours/12000cycles. in calander age the boeing was newer, but in airframe life the fokker is 4 times older.
For years there have been structural mods on lapjoints on Boeing, especially the 737 series (good little earner), most of them come to light during ageing aircraft inspections.
But still there are people in the trade who think it's not a problem to use stanley blades and the like on a pressurised hull.....