Renesmee will never reach adulthood she stops aging at age 17 like her father
-- The only actress who will play Renesmee is 10 year old Mackenzie Foy.
Breaking Dawn 2 ends where the book ends...when Renesmee is about 18 months old but looks 5 years old. They made her older in the film because it's too hard working with 5 year olds. The movie used body doubles and CGI to progressively age Mackenzie Foy until her actual body is used at the end of the film. They wrapped when she was still 9 years old. ( Nahuel said he stopped growing at 7 years and looks like he is in his late teens although he is about 150 years old).