Large old hollow-bearing trees have a wide range of key ecological roles in forest and other ecosystems globally.Patterns and rates of mortality and decay of these trees had profound effects on the size and composition of theirpopulations. Using an 18-year empirical study of large old trees in the Mountain Ash (Eucalyptusregnans) forestsof the Central Highlands of Victoria, we sought to determine if there are particular patterns of decline that areshared by a proportion of the trees in a tree population. We also sought to identify drivers of decline of thesetrees by quantifying relationships between the condition state of trees (viz: tree form) and a range of covariates.We found that time, stand age and fire can individually and in combination, strongly affect the decay (andeventual collapse) of large old trees. In particular, we found compelling evidence that patterns of tree declinewere markedly different in old growth forest (stands dating from ∼1850) relative to three other younger ageclasses examined. Trees in older forest decayed less rapidly than trees of equivalent tree form in younger forest.Old growth stands also were characterized by trees in an overall much lower (more intact) form category thanthe other age classes of forest. A key pattern in our study was the rapid deterioration of large old trees in theyoungest aged stands (viz: those regenerating after fires in 1939 and following disturbance between 1960 and1990). In these forests, a very high proportion of large old trees were either in the most advanced state of treedecay (form 8) or had collapsed (form 9). This is a major concern given that 98.8% of the Mountain Ash forestecosystem supports forest belonging to these (or even younger) age cohorts. Our investigation highlights theneed for forest management to: (1) increase levels of protection for all existing large old hollow-bearing trees, (2)expand the protection of existing regrowth forest so there is the potential to significantly expand the currentlyvery limited areas of remaining old growth forest.