  Neutral models for the dynamics of a system of competing species are used,
nowadays, to describe a wide variety of empirical communities. These models are
used in many situations, ranging from population genetics and ecological
biodiversity to macroevolution and cancer tumors. One of the main issues
discussed within this framework is the relationships between the abundance of a
species and its age. Here we provide a comprehensive analysis of the
age-abundance relationships for fixed-size and growing communities. Explicit
formulas for the average and the most likely age of a species with abundance
$n$ are given, together with the full probability distribution function. We
further discuss the universality of these results and their applicability to
the tropical forest community.
