1. Understanding the relationship between age and size at maturity is essential because these traits are pivotal determinants of an organism’s fitness.
2. The relationship between age and size is commonly addressed using optimization and quantitative genetic approaches. Here we argue that the value of such studies is often limited by an insufficient consideration of organismal ontogeny.
3. On the basis of a simple conceptual framework of hierarchical resource allocation, we identify key aspects of ontogeny that prove critical to a fuller understanding of the relationship between age and size, and which, to date, have been insufficiently explored. In particular, these include intrinsic variation in growth rate within and among populations, and the physiological nature of the maturation process that co-ordinates growth and reproductive function in an organism.
4. We also provide some guidance to the empirical investigation of these aspects, anticipating that a wider theoretical, but especially empirical appreciation of ontogenetic detail will greatly increase the explanatory and predictive power of life-history studies.