Cracks Developing in Fiberglass

Simple age. I traded my last Duly pickup truck for the same reason. Each side of the truck bed was one continuous fiberglass. For some reason, the fat fenders started cracking out. I attempted to get them repaired, they said, no, the entire side needed replaced, at $4000 a side, I said "No way!" I traded the truck while it still have value, and before a chunk of the fiberglass fell off, for a new one (metal sides now, so now I can watch them rust!).

Reason for the cracking? just age.