What really grabs me, visually, is her tremendous capacity for expression which appears to be drawn from a rich pallet of subtle and nuanced elements that somehow she combines into a beautiful and fluid portrayal of the character that she brings to life.
Indeed! Just look at every role she played so far. If somebody would convert the footages to pictures that'd show that she uses a wide range of tiny expressions with her face and body. A little smile on the edge of her mouth, a slightly different pose of her body, they make a difference. Or nice touches like when she scratches her face as Skyler sending the message that she is a roqueish girl who doesn't care how well mannered she looks like.
These are small things which are sending a message to the audience about the emotional status of the character along with the spoken lines creating a strong connection with the viewer and makes the scenes more powerful.
This technique is missing with most actors nowadays and this is a reason why Summer is so refreshing to watch.
I agree that her expression makes her look young. (Actually, I think she looks a little zonked-out in this pic.) But to me her face looks more like the face in post #4 than any of the faces in post #2.
Yeah, its just her expression probably, but maybe this indicates that the theory that we see her more mature recently because of her expressions matured somehow may be somewhat valid. She can look younger with a less mature expression so the change shouldn't be that severe physically.