This is the number of years that someone has lived, or that something has existed.
(What is the part of speech of 'that'? Is 'that' a relative pronoun or a relative adverb? Can we use 'when' instead of 'that' in the sentence?)
English - Writeacher, Tuesday, November 30, 2010 at 7:03am
In that sentence, "that" is being used as a relative pronoun, referring to "years" as its antecedent.
If you use "when" instead of "that," you change the meaning of the sentences.