Leaf senescence contributes to reduced canopy photosynthesis when new leaf initiation cannot compensate for increased leaf aging after canopy closure. However, since leaf aging is seldom addressed in whole-plant investigations, it is difficult to evaluate how age-class composition influences crop productivity. Therefore, field studies were conducted to examine leaf area development in cotton (Gossypium hirsutum L., cv. S_506) and to incorporate this information into a simulation model for assessing canopy age-class dynamics. Simulated results indicated that leaf area produced during rapid canopy expansion (60 DAP, LAI=2.3) was largely composed of young leaves with over 35% of the LAI being less than 14 days old. However, 30 days later (90 DAP, LAI=4.5) leaf area production was no longer sufficient to offset leaf aging and the percentage LAI less than 14 days old decreased to below 10%, while leaves greater than 29 days old increased to over 75%. This shift in age-class composition towards progressively older leaves altered the relative contribution of carbon from differing leaf layers within the canopy. Using model-based estimates of carbon assimilation and patterns of leaf area distribution, the middle portion of the canopy was the most important contributor of carbon early in crop growth, supplemented by roughly equal contributions from the lower and upper canopies. However, after 90 DAP the supply of assimilate from the upper canopy rapidly increased to over 45% and that of the lower canopy decreased dramatically. Towards the end of the season (120 DAP) the middle and upper portions of the canopy were about equally divided in their importance to late-season carbon production. These results, while identifying what we believe is a potential limitation to yield in this crop, can assist producers in better understanding how changes in age-class composition impacts cotton production.