Aluminum definitly suffers from fatigue. But I'm not sure how to tell if an aluminum frame/fork is fatigued.

And carbon, I worry about UV light from the sun and how it affects the hardner in the resin matrix between the carbon fiber. Plastics made with hardner will cause the plastic to become hard and brittle over time. Sometimes its accelerated by exposure to UV. And there is the problem with oxidation even on plastics.

I honestly don't know about how long a carbon fork will last. If I see a hairline crack in the resin, it's time to replace. you might want to pour dye, with different color than the fork, on it so you can see the crack.

Food for thought: I've seen aluminum frames fail. My steel frame using tange #1 tubing failed at the bottom bracket, Then again I had about 15,000 hard miles on it. And just about two weeks ago, I saw my first cracked Ti frame with 3/2.5 composition. It cracked on the downtube right where the cable guides attach near the head tube, a Lemond bike. I think it had over 20,000 hard miles on it. male cat 4 rider.
And yes I've also seen carbon frames fail too but it was one of the earlier(older) frames.

smilingcat