Density dependence plays an important role for population regulation in the wild. It involves a decrease in population growth rate when the population size increases. Fifty years ago, Charlesworth introduced the concept of "critical age group", denoting the age classes in which variation in the number of individuals most strongly contribute to density regulation. Since this pioneering work, the use of this concept has remained scarce. In light of Charlesworth's concept, we discuss the need to develop work between behavioral ecology, demography and evolutionary biology to better understand the mechanisms acting in densityregulated age-structured populations. We highlight demographic studies that explored agespecific contributions to density dependence and discuss the underlying evolutionary processes. Understanding competitive interactions among individuals is pivotal to identify the ages contributing most strongly to density regulation, highlighting the need to move towards behavioral ecology to decipher mechanisms acting in density-regulated age-structured populations. Because individual characteristics other than age can be linked to competitive abilities, expanding the concept of critical age to other structures (e.g. sex, dominance rank) offers interesting perspectives. Linking research fields based on the concept of the critical age group is key to move from a pattern-oriented view of density regulation to a process-oriented approach.