As plants age, they go through developmental transitions that impact their form and function. These transitions include the often visually subtle change between juvenile and adult vegetative stages known as vegetative phase change (VPC). How plants respond to dynamic challenges in their environment varies with age and has important implications for plant performance and fitness. To gain more insight into what role VPC has on these age dependent processes and its importance for plant functioning, this thesis explores changes in ecophysiological traits of juvenile and adult developmental phases. Across these studies, overexpression of miR156, the master regulator of VPC, was used in three species—Populus tremula x alba, Zea mays, and Arabidopsis thaliana—to delay the timing of VPC and differentiate the effects of developmental programing from other confounding factors such as plant size. Photosynthesis, a dominant driver of plant success during vegetative growth, is found to be phase-specific across species. After an extensive biochemical analysis yielded no phase-specific differences, I identified leaf composition traits, specifically, specific leaf area and nitrogen distribution, as the drivers of these developmental differences in photosynthesis. These photosynthetic and compositional differences lead to opposing carbon economic strategies between juvenile and adult leaves of all three species. Adult leaves cost considerably more to build than their juvenile counterparts and require more time to assimilate enough sugars to payback these costs. However, the longer adult leaf lifespan results in a greater return on investment than juvenile leaves. When modeled across varying light environments, the economic strategies of juvenile leaves are better suited for low light conditions while adult leaves benefit significantly under high light conditions. Overall, I demonstrate that juvenile leaves are photosynthetically distinct from adult leaves and that this difference can be attributed primarily to morphological differences. This yields two carbon economic strategies – a low-cost low-return juvenile and high-cost high-reward adult strategy. My findings indicate VPC is responsible for differences in plant function and physiology observed across their lifespan. I provide evidence that juvenile and adult tissues are advantageous in different environments and the timing of this transition could be an important target for selection.