#1 Rule - ANY cuts, even nicks and the chaps need to be replaced. Any weakened threading decreases the protection of the chap as a whole.
As far as age, dirt,etc.., Outside of getting specifications for each type of thread and cloth from the manufacturers, it all depends on individual use and care. An analogy would be your car tires for example: How fast do your drive? Do you drive in the cold or do your tires sit in the hot sun? Do you clean and treat them regularly?
I will do some research into the specific materials for you.