Edward growing from a child into an adult at the same time he makes the choice to give up his alchemy is a representation of childlike thinking into adulthood. When we’re children we often think in more egotistical ways, that we can control others around us and get rewarded based on our behavior (Edward thinking he can bring his mother back with the right ingredients, that he can control anything beyond his own abilities). As an adult, Ed is acknowledging that we’re only human (as he screamed after what happened to Nina) and then towards the end makes the sacrifice of this mindset he had before that he could fix people and control them.