The phrase “When My Brother Was an Aztec” is the title of a poetry collection by Natalie Diaz, not a historical time period someone literally lived in.

Meaning of the phrase

In Diaz’s book, the brother is compared to an Aztec god and to mythic figures as a metaphor for his power, destructiveness, and the way his addiction reshapes family life. The “Aztec” part invokes imagery of sacrifice, gods, and intense conflict rather than saying he was literally part of the Aztec civilization.

If you meant history

Historically, the Aztec (Mexica) civilization flourished in central Mexico roughly from the 1300s until the Spanish conquest in 1521. So no one alive today would literally have been “an Aztec” in that original imperial sense.