Here's my take on all this, as someone with no formal experience.
The rise of the international blockbuster over the last five years has meant that studios are making huge amounts of money on foreign territories. But this has also led to a misconception that a movie's domestic gross is irrelevant, which is not the case.
In the last couple years we've seen several "quasi-flops," a phrase I just made up that refers to a movie that underperformed (or flopped) domestically but featured enough CGI explosions to turn a small profit internationally. Some examples are The Amazing Spider-Man 2, Star Trek Into Darkness, and Man of Steel.
Now, is Age of Ultron underperforming or a flop? Absolutely not. But it's not reaching the heights it was originally predicted to, and that is certainly some cause for concern as we watch the future. In the above three examples, the movies may not have been huge financial catastrophes, but the studios realized that they needed to do something fast or the next film would be.
As a few others have said, Age of Ultron isn't the one we should be watching. Ant-Man is. If Ant-Man underperforms (or worse, flops), Marvel will be certainly be rethinking some of their Phase Three plans. If Ant-Man can't hold his own movie, how can Black Panther or Captain Marvel? And please, don't cite Guardians of the Galaxy. Just don't. It's not remotely the same type of film and there are about 27 different variables that helped make it a success that the other standalones don't have.
So tl;dr: don't worry too much about Age of Ultron's gross. Worry about Ant-Man's.