Yeah, Marion was quite clearly jailbait when she first hooked up with Indy.
According to the Raiders script, she's 25 during the film itself, and the affair was ten years beforehand. So she was 15.
However, the later chronology in stuff like the Ultimate Guide (by the way, is it still canon, what with all the SW universe shenanigans?) fudges Marion's age at the time.
She's bumped up to age 17, in an effort to wipe away this unpalatable fact as much as possible--while remaining unnoticed to the casual observer. GL really is devious when it comes to subtle alterations to his work.