We quantify the relationship between forest stand age and fire severity using a detailed case study of Mountain Ash (Eucalyptus regnans Muell) forest burned in south‐eastern Australia in 2009. We focused on two important areas of Mountain Ash forest that feature a range of growth stages and disturbance histories. Using probit regression analysis, we identified a strong relationship between the age of a Mountain Ash forest and the severity of damage that the forest sustained from the fires under extreme weather conditions. Stands of Mountain Ash trees between the ages of 7 to 36 years mostly sustained canopy consumption and scorching, which are impacts resulting from high‐severity fire. High‐severity fire leading to canopy consumption almost never occurred in young stands (<7 years) and also was infrequent in older (>40 years) stands of Mountain Ash. We discuss the significant forest conservation and management implications of these results for Mountain Ash forests as well as other similar biomes, where high‐severity fire is a common form of disturbance.