Why do some substances age when exposed to light for a long time (such as plastics), while others have no effect (such as steel)? What is the scientific truth?

Polymer compounds such as plastic fibers contain unsaturated chemical bonds such as C = C = O. Under ultraviolet irradiation, these chemical bonds are unstable, easy to break and undergo irreversible chemical changes, leading to material aging. However, the atomic bond between steel and iron can only be broken and dissolved at extremely high temperature, and light hardly affects it.