The following is a rumored casting call for extras supposedly released by Marvel Studios and posted on the website Feature Film Casting:
[Possible spoilers ahead]
"Marvel Studios and The Walt Disney Studios are now in pre-production on the upcoming sci-fi themed superhero feature film “The Avengers: Age of Ultron", and casting directors are auditioning actors for starring, co-starring, and supporting roles. The film’s photo doubles and stand-ins for the starring actors, as well as general extras and featured background actors will be cast closer to the start of filming. The locations where outdoor principal photography will take place are in and around London, England. Indoor sound stage filming will take place at Shepperton Studios. Shooting begins in February, 2014.
In “The Avengers: Age of Ultron” Loki is gone and the new villain will be the evil robot Ultron, who has the ability to keep upgrading his own strength, making him extremely fearsome. Tony Stark/Iron Man becomes the unwitting creator of Ultron. The brother-and-sister team of Quicksilver and the Scarlet Witch become the seventh and eighth members of The Avengers. Quicksilver has extreme speed, while Scarlet Witch can cast spells and has telekinesis ability. Black Widow and Hawkeye will be key figures in the film.”
It doesn’t sound particularly official, as though it was pieced together using random information online (plus some of the information on the website appears dated).
In any case, consider it with a huge grain of salt.