At age 24, Scarlett Johansson started her role as Black Widow in Marvel Cinematic Universe Phase One's Iron Man 2 working undercover as Stark's assistant and she saved Rhodey from Ivan Vanko's control over the War Machine Armor. She even saved Stark from dying.
In the Avengers 2012 film as one of the first 6 Avengers. Surely she is official due to her membership in the Ultimate Avengers movies but here in the Marvel Cinematic Universe, she and Hawkeye take Wasp and Ant Man's places as the two other female and male Avengers along side the iconic 4.
In the Upcoming Captain America: The Winter Soldier, she teams up with Captain America to hunt down a mysterious enemy and it is rumored that the two characters will be starting out their relationship.
Now in Avengers Age of Ultron, Whedon said Scarlett's role will be significant. Not sure what he means by 'significant' but it must mean by being the main hero, main character or the person who is going to defeat Ultron. Anyway, Scarlett and the other cast members will reassemble in the sequel with Aaron Taylor Johnson joining them as Quicksilver and Scarlet Witch whose actress that hasn't been announced yet.
More hints on the film will be revealed in time before the first trailer hits the web.