First off, it is "relative pronoun" (關係代名詞), not relative adverb - I doubt there is such a thing as relative adverb.
And yes, the relative pronoun "[when/in which]" relates to "an age". Great ice sheets existed in that age, which is natural and exact than ice sheets existing in the concept.
And again yes, "when" can be used with anything that has some notion of time in it. Ex: "Those were the good old days when I didn't have to work so hard". "That was the moment when[that] I realized I was alone". Neither of these has the word "time".