If you are writing a story that isn’t linear and compact in time then you are going to need visual, verbal and audible cues to the audience when you want to move from one point in time to another.
In Juno, Autumn, is written on the screen in the opening scene and the time frame is now established. We can now easily skip, as this story does, forward several months into the future without disorientating the viewer. Do you notice the font and style of the writing ‘Autumn’? it’s written like a child or teenager, crayon style. Diablo Cody is establishing (although this was probably done by Reitman or a production designer) that this is a youth/teenage story from the opening frame.
The environment is established very quickly by the opening shot of suburban American houses. This is not a fantasy movie or story about rich people in Beverly Hills, it’s a story of a middle class teenager, just like me or you, in a normal neighbourhood.
Can teenage youth be more summed up than hoodies, jeans and a gallon of Sunny D?