Ontogenic development is divided into infant, juvenile, adolescent and adult life-stages. Although the developmental trajectory of an individual is a flexible entity, which differs within species, environment and sex, life-stage classifications are generally structured, age-based systems. This invariably leads to rigidity within a dynamic system and consequently hampers our understanding of primate life history strategies. We propose that life-stage classifications should be quantitative, flexible entities, which use a reliable measurement of development. Here, we provide a methodological example where placement into a life-stage is based upon behavioral variance between other similar-aged individuals. Behavioral data were collected from 12 male (3–11 years old) and 9 female (3–8 years old) captive immature western gorillas (Gorilla gorilla gorilla) housed in five family groups, using continuous focal sampling; 900 hr of data were collected over 131 days. Data were applied to four published life-stage classifications for mountain gorillas (Gorilla beringei beringei), which showed variable ability to determine life-stage in western gorillas. A new life-stage classification (Hutchinson & Fletcher) was proposed specifically for western gorillas, whereby multiple co-varying behavior provided a robust measure of linear development across immaturity. Each life-stage was found to be a distinct ontogenic phase and the classification discriminated life-stage with a high level of accuracy. Using the Hutchinson & Fletcher classification we provide evidence for disparity in developmental trajectories between the sexes from the juvenile period onwards. To expand the understanding of primate life histories, we propose that flexible classifications should be used to enable comparison of allometric life history traits within and between species, from birth onwards. Am. J. Primatol. 72:492–501, 2010. © 2010 Wiley-Liss, Inc.