"Passed" versus "past" instance in a published novel

Past is an adjective in this case, not a noun.

Past, adj: Gone by in time and no longer existing. "The danger is now past."

Compare this to "The age is just past." Past is used the same way in the text; as such, the original version is a correct usage.

The new version isn't technically incorrect either, however, though it would be less awkward as "the one that just passed". But certainly, the author did not make an error in the original version. If it wasn't corrected by the author, then it was a mistake to change it.

As per this answer to a related question, past refers to a state. The state of the older age is over, bygone, or past; not passed. The state of the new age is next or upcoming, so the use of "past" sets up a contrast. "Just passed" doesn't do it quite as well.

past is a noun.

Noun: The time or a period of time before the moment of speaking or writing.

passed is a verb

Verb: Move in a specified direction: "he passed through towns".

So the newer version is correct.

"[..] a message for you about the next age, and the one just passed."

The sentence is saying that the message is about the next age, and about the age of time that just passed (moved) as the present.