After hearing about the twist and looking closely at the picture of Esther realizing "Holy shit she does have age marks!"
There's a lot that will make you say "holy shit!" on a second viewing. Why does she prefer to wear Victorian dresses over blue jeans? To hide her more "adult" figure. Her avoidance of doctor and dentist appointments. "I've had a lot of time to practice, I guess." "She's very mature for her age." There are even subtler moments like when Kate is yelling for Esther to hurry up on her first day of school...perhaps she was taking so long because she was putting on all her makeup.
Here's a subtle one: Danny notices Esther cuts her food into little pieces before eating anything. It could just be her style of eating, but it makes a lot more sense when you find out that she's wearing false teeth and eating with them in would probably be very difficult without cutting things very small.