centuries or century

[Hung Tao]

Please explain why the word "centuries" is used in plural form in the following sentence:
It reserves centuries-old thinking and is a rare concession to the power of nature.
"centuries" in the sentence above is used as an adjective so it should be in singular form like "a seven-year-old boy".Please explain this grammar point.
Thanks a lot.

[Alan]

The use of the singular form for attributive nouns is a general tendency, not a rule. 'Century-old' would mean exactly one hundred years old, and is thus semantically different from 'centuries-old'.