In Our Mrs. Reynolds Mal is purportedly married to Saffron. The Shepherd tells Mal:
If you take sexual advantage of her, you're going to burn in a very special level of hell. A level they reserve for child molesters and people who talk at the theater.
While she certainly seems innocent and frightened to some degree, and I would agree that taking advantage of that situation would be immoral, I don't see why he would be so sure she was underage (i.e. a child).
I'm not sure when the episode was actually shot, but if we assume it wasn't shot more than one year before it aired, that leaves us at 2001, meaning Christina Hendricks was about 26. Again, the con artist (whose name we never learn) who played Saffron did a tremendous job (and so did Christina in playing her) in fooling the crew, but she certainly didn't look like a child.
So, how did the Shepherd figure she was? Is there maybe even a source that states the age of the character? Does it differ largely from the age of the actress?