Back in August, it was confirmed that Bradley Cooper and Elizabeth Olsen are in talks to play Quicksilver and Scarlet Witch in The Avengers: Age of Ultron. While we haven't officially heard if those deals have been finalized yet, Samuel L. Jackson essentially confirmed in a recent interview that Elizabeth Olsen has signed on.
"I don't think we begin shooting before March of next year. I know we're shooting in London, that James Spader is Ultron and going to be the bad guy, and that we added Ms. [Elizabeth] Olsen [who will play the Scarlet Witch], but I don't know what she's doing, if she's on the inside or the outside. I haven't seen a script."
Joss Whedon is directing from his own screenplay, with Marvel's The Avengers stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson and Jeremy Renner expected to return.