DerekJ_MI
How long do they last?  Should I only consider a frame that is a few years old or one that has very limited use/miles on it?  I want to build up an inexpensive road bike and I see many CF frames for sale but some are 12 to 15 years old or older. They look good and the price is right but I've heard that there can be micro cracks that develop over time.  I've also heard that CF has changed and that the newer frames, made in the past 5 years or very durable, but not so much so for the older frames.  What is the consensus on this?  I don't want to be buying someone's problem.
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chas
Mine is 25 years old, fell off a car at 80mph on I-75 about 12 years ago, and is doing fine (the aluminum bits were trashed after hitting the road at 80mph).  

Typically Carbon doesn't like point loads, but a well designed one can last a good long time.
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67flieger
In general, I think good advice with any carbon frame that you don't know the full history of, is to have it thoroughly checked out by a qualified tech/mechanic.  More so than age, I'd be concerned about hidden damage.  Carbon fibre can fail in ways that aren't visible from a cursory look over the surface.
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DerekJ_MI
I talked with my LBS and he said the rule of thumb is 5 to 7 years of riding 5,000 miles a year.  He said that the issue is internal micro cracks that are difficult at best to see and you won't see them on the outside of the frame.  He recommends that if you buy used, buy a frame that is low millage and only a couple of years old.  Otherwise choose Aluminum, steel or TI.  I have a Lynskey gravel bike and I bought it for the durability I wanted on gravel. This bike though is for pavement and maybe use in the Kickr trainer.
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bnystrom
If you have a particular frame in mind, do some research online to see if there are any common complaints about it. If not, find out as much as you can about its usage hisrory from the owner and inspect it carefully before you buy it. Also, keep in mind that on a bike that old, you're probably going to have an 8 or 9-speed drivetrain and finding repair parts could be an issue. Also, if it has a replaceable derailleur hanger, they may be hard to find, too.
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