Age-based methods to sustainably hunt require precise age estimates from appearance.

Used photos to track trait change with male lion age and test hunter aging accuracy.

6 traits precisely indicate 4 age classes: 1–2.9 year, 3–4.9 year, 5–6.9 year, ≥ 7 year.

Hunters used traits to accurately age 69% of lions, with fewest errors for ≥ 7 year.

Min harvest age of 7 year with strict enforcement could enable sustainable harvest.