Age-size relationships of a species in any given population are variable due to local environmental and genetic variations across individuals. The aim of this study is to test an age-size model for the keystone Carnegiea gigantea (saguaro, Cactaceae), that establishes in cohorts, to assess its accuracy in reconstructing those cohorts. Monte Carlo simulation is used to generate a Carnegiea gigantea population based on parameters selected and then applies the age-size model to the population to ascertain its effectiveness. Individuals in a cohort of different sizes are generated, as would be expected in the real world, and a simulated empirical dataset is created. Variation in growth over time incorporates two sources of variability, (1) individual variability (e.g. genetic or microsite variations) as well as (2) population-wide variability (such as fluctuations in rainfall from year to year). Generally, older cohorts are more difficult to accurately estimate, but all cohorts are identifiable. Results suggest that the Drezner model for Carnegiea gigantea is robust for reconstructing periods of establishment. This test of the Drezner model using annual and individual multipliers can be applied to other age-size models to ascertain their effectiveness, particularly for cohort identification.