In the end of The Twilight Saga: Breaking Dawn – Part 2 Alice's vision shows the grown up Renesmee. So how this scene got achieved? How they managed to show a young Renesmee to a mature Renesmee?
On the IMDB page, there are a number of actresses listed as playing Renesmee at various points in her growth including one credit for "Christie Burke ... Renesmee (Young Woman)".
So it seems unlikely that any special effects were used.