You start off as a child, and you can do things that kids can do. You allocate how much time and energy you want to devote to various projects (e.g. study for math class! Woo that cute girl!) Then as you get older, the things you can allocate energy to are based on what stage you are in life (adults can do different things than high school students!), and based on what you did when you were younger (if you woo'd that cute girl as a child, maybe you can marry her when you are older!)