When two things live or happen during the same period of time, they are coeval. If you annotate an old poem, the annotations and the text of the poem are not coeval.
The word coeval comes from the Latin co- "jointly" or "in common" and aevum "age." The beginning of Major League Baseball is coeval with the invention of the telephone. People can be coeval, though more often you'll hear contemporary used to describe people who are about the same age. You and your contemporaries probably view the world a lot differently than your grandparents' generation.