hundreds of years....

The correct form is "hundredS OF years", but the interesting issue is: why?. The answer is this: "hundred" belongs to two syntactic categories, noun and numeral. The noun "hundred" 1) is a count noun, 2) is used ONLY IN THE PLURAL, "hundreds", 3) may be followed by prepositional phrases headed by "of" (cf. hundred(s) OF students, not *hundreds students), but 4) CANNOT be followed by noun phrases (cf. "*hundreds students" is incorrect), and 5) CANNOT be preceded by numerals, cf. "*two/three/five HUNDREDS (of students)". In its turn, the numeral "hundred" 1) is invariably SINGULAR, 2) MUST be preceded by the numerals "one/a", "two", "eight"... or by a quantifier like "many", "several", "a few", 3) must be followed by NOUN PHRASES (count, plural), and 4) CANNOT be followed by "of" + noun phrase. Thus, "three hundred students", "several hundred students" are OK, but "*three/several hundreds students" is wrong, and so is "*three hundredS OF students."

hundred of years ( because hundred no plural