Explanation:

- A birth group: Not a commonly used term. Generally refers to individuals born around the same time but is a vague concept.

- A life stage: Refers to phases in an individual's life, like childhood, adolescence, adulthood. It’s not specific to birth years but more related to experiences and development.

- An age cohort: A demographic group of people born during the same period. Often used in sociology and research to study patterns and behaviors of similarly aged people over time.

- An age level: Generally refers to maturity or development stages. It’s not strictly connected to birth years as "age cohort" is.