Yes, the plastic chain rails definitly are age sensitive. They turn hard and brittle over time and exposure to hydrocarbon vapors (my own interpretation). Mileage and heat cycles and oil changes also matter. But age does them in.
An experienced MB tech can tell how "well done" they are by looking at the color. Anything beyond soft amber scares me. They are semi-translucent white to start with.
Bosch Master, ASE Master, L1
33 years MB technician