Guide me then, which source tell you otherwise.You need to re-think what you said there.
Aerospace industry seems to have fatigue life of material figured. Aluminum parts have pretty low fatigue life so it usually design to not flex to minimize its fatigue cycle. Its strength degrade at quite a fair bit higher rate than steel and titanium, while carbon fiber can flex over and over without losing integrity (as long as you don't cross the line and destroy it).
You can also see example of ancient aluminum road stem completely break apart from fatigue after a decade of use. My 10 years old aluminum road frame is also no longer feel stiff. I guess micro cracks are all over the frame and weaken it significantly by now.
You won't notice this on carbon fiber that even design to flex (which, in theory, use up a lot of fatigue life), or only after a life time on titanium and steel frame.