Exactly what dialog? Because (based on my recollection) if there was any dialog that implied that, it was ambiguous or must be the kind of thing you'd have to be paying pretty careful attention to to pick up on. So close attention that you're probably ignoring the actual plot of the movie.
M: I learned to hate you in the last ten years.
I: I never meant to hurt you.
M: I was a child! I was in love! It was wrong and you knew it!
I: You knew what you were doing.
M: Now I do! This is my place. Get out!
I: I did what I did. You don't have to be happy about it, but maybe we could help each other out now.
https://youtu.be/Q4PlXSw0BQo?t=118
It's ambiguous (since we don't know for sure how old Marion is during the events of Raiders), but "Marion was 16(ish)" is a perfectly valid reading.
Yeah, which is kind of my point. That may be a valid reading of the dialog, but so is 18, 20, or even 25. 16 may be in the movie's "writer's bible," but (IMHO) the viewer has to do too much work during that 10 seconds or so to get there. A regular viewer isn't paying much if any attention to the character's ages, and pretty much all of them will assume something legit.
G — I was thinking that this old guy could have been his mentor. He could have known this little girl when she was just a kid. Had an affair with her when she was eleven.
L — And he was forty-two.
G — He hasn't seen her in twelve years. Now she's twenty-two. It's a real strange relationship.
S — She had better be older than twenty-two.
G — He's thirty-five, and he knew her ten years ago when he was twenty-five and she was only twelve.
G — It would be amusing to make her slightly young at the time.
S — And promiscuous. She came onto him.
G — Fifteen is right on the edge. I know it's an outrageous idea, but it is interesting. Once she's sixteen or seventeen it's not interesting anymore. But if she was fifteen and he was twenty-five and they actually had an affair the last time they met. And she was madly in love with him and he...