Aluminum components do fatigue but it is a matter of miles ridden and rider strength rather than time. In the days of steel bike frames pro teams routinely replaced cranksets, stems and bars at least annually or after a crash to prevent fatigue failures. Remember though a professional rider is using the components in question much harder than your average rider.
I suspect that with CF bikes they are replaced after any accident which might have damaged the frame or fork whether damage is clearly visible or not. A bike is cheap compared to a professional rider's salary and/or medical costs due to a component, frame or fork failure.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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