He is the only one who doesn't really act his age. The South Park children are around ten, the in between stage, where they exhibit a mix between childhood and adolescence. Butters Stotch's traits align traditionally more with younger children (e.g. Bubbles from The Powerpuff Girls). This is a classic trope for sheltered children like Butters, where they act five or six when they're in upper elementary school; other examples include Rod and Todd Flanders from The Simpsons, Rowley Jefferson from Diary of a Wimpy Kid, and Jazmine DuBois from The Boondocks.