In iOS 7 UITextView is based on TextKit and has a new property textContainerInset. It behaves just as you would expect:
UITextView *textView = ...;
// Left inset of 50 points
textView.textContainerInset = UIEdgeInsetsMake(0.0, 50.0, 0.0, 0.0);
Swift 4.2
textView.textContainerInset = UIEdgeInsets(top: 0, left: 50, bottom: 0, right: 0)