years vs period of time

years and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
years	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of years; that is, period of time is a word with a broader meaning than years:

Other hypernyms of years include period, time period.

years (noun)	period of time (noun)
a late time of life	an amount of time
a prolonged period of time
the time during which someone's life continues