The major goals of this cooperation project were to examine whether a common experimental design can be conducted for the study of lifespan changes in cognitive flexibility and to determine the influence of language processes and environmental cues on lifespan changes in components of cognitive control processing. Overall, we run seven common experiments in which we primarily used the task switching paradigm to investigate age-related changes in cognitive control from early childhood to old age. In order to find an appropriate paradigm, we assessed, in a first step, whether the factor stimulus set-size modulates the measurement of lifespan changes in cognitive flexibility (as indexed by mixing and switching costs) and inhibitory control (as indexed by interference costs and the Gratton effect). Results suggest that the number of different stimuli primarily influenced inhibitory control and this influence varied with age. Using only a few stimuli for the measurement of task switching produces stronger taskstimulus priming than using a large stimulus set-size, and this increased the need for inhibitory control primarily for elementary school children, interestingly, kindergarten children were not susceptible to variations of set size, possibly because they were less efficient in representing higher-order task goals. Thus, results of Experiment 1 reveal that stimulus set-size has an impact on the interplay between automatic driven processes and higher-order control processes whereas older children were primarily influenced by variations of associations between tasks and stimuli. Therefore, for the study of lifespan changes we used only larger stimulus set-sizes in the following experiments. In the second step, we run several experiments to determine the specific function of language processes as a possible cognitive intervention to reduce age differences in cognitive control. Our results indicate that all age groups profit from verbalizing the next upcoming task (goal labeling) prior to target presentation (i.e. during task preparation), and that this strategy is most efficient when task switching is already practiced as the additional verbalization also puts demands on working memory. Thus, task naming is a useful tool by supporting the optimal task-set retrieval. Interestingly, older and younger adults use this strategy independently of environmental support (spatial task cueing), while there is partly evidence that children use verbalizations less when environmental prompts are present. We furthermore showed that verbalizations are even beneficial in early middle childhood and we ruled out that verbalization benefits simply reflect a general attentional orientation function. We also investigated the effects of different types of verbalization (goal vs. response labeling) during target processing (when cues and targets are presented simultaneously). Here we found that labeling in general promotes the implementation of a task switch only in the elderly but not in children and hinders it in young adults. Taken together, we interpret these findings in light of theoretical models assuming age-related changes in different cognitive control modes (cf. Braver's model): proactive and reactive control. Young adults have a tendency to strongly engage in advance preparation, which can be supported by goal labeling during task preparation but labeling during target processing conflicts with response selection. Children and older adults have a tendency to reactivate the task cue information during target processing. Hence, goal labeling promotes proactive control during task preparation but also facilitates the reactivation of task cue information during target processing, at least for the elderly. To further evaluate this theoretical view, we finally assessed the infiuence of cue vs. target verbalization more directly with a different paradigm, a modified version of the AX-CPT task. Results of two experiments with different stimulus materials show that cue verbalization support proactive control and target verbalization reaction control in children, younger and older adults. To conclude, language supports task preparatory processes, that is, the retrieval and maintenance of the task (cue) information (a proactive control style), therefore especially children and older adults profit from goal verbalizations. For older adults, in contrast to children, verbal cueing is beneficial, even if other cues are available and they also profit from labeling during target processing. Hence, language is a powerful self-cueing device, and by this, enhances the flexible adaptation to changing task demands.