Treizade
Western culture typically goes by units of 10s, counting with toes and fingers. Aztecs went by units of 13s, counting with moon cycles. Instead of a decade (ten years), an Aztec would typically measure time in a treizade — a unit of 13 years.
I am 29 years old now. I am not in the final year of my third decade of life, I am in the third year of the third treizade of my life.