They got her early. She had a few solid credits to her name when she did Iron Man 2, but the smart MCU move at that point was to recruit young-ish talent and get them signed on for multi-movie contracts relatively cheap. Hopefully what we all get out of that sort of arrangement is that talented actors make enough MCU money to go off and do whatever out-there artsy stuff that creatively appeals to them and fosters their talents, rather than having to plug away in garbage movies just to pay the bills.