In order to assess the effects of age on flooding tolerance, we conducted a mesocosm flooding experiment that examined biomass accumulation of two age cohorts of Xanthium strumarium L. We also examined repeated measures of stem height and diameter of each cohort for the duration of the experiment. We hypothesized that change in biomass measures (root and shoot biomass, increase in total biomass, and root shoot ratio) of older individuals would be greater than younger individuals under simulated flooded conditions. We further hypothesized that older individuals would be impacted more than younger individuals regarding stress; i.e., the younger cohort would acclimate and thus flooding effects would be masked. Results indicate that older individuals were more impacted by flooding than younger individuals for all biomass parameters. Height repeated measures indicated that older individuals differentially responded to flooding stress, compared to younger individuals; which was also the case for diameter repeated measures. Our study indicates that individuals exposed to flooding at an early age could be plastically developing acclimation structures leading to younger cohorts subsequently outperforming their older cohorts that did not develop under the stressed conditions.