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.This is alluded to in the film, but not so explicitly. Marion tells Indy "I was a child. I was in love. It was wrong and you knew it." The film isn't explicit about ages, though. I always assumed Marion was saying "I was naive", not "I was jailbait."
The ages of the actors don't really prove whether the encounter was illegal or just unhappy. Karen Allen was 28 (birthday 10/05/51) when Raiders was filmed (06-09/80), Harrison Ford was 37-38 (birthday 7/13/42). As Lucas wanted, they are ten years apart in age. How old were they when they got it on? Marion says "I've learned to hate you in the last ten years." Ten years prior, Allen was 18, but it's not certain whether she's being precise or playing her actual age. Seems, though, like she's on the safe side of legality there.
I suppose that it's a matter of changing standards, but I'm surprised that Lucas would flirt with the possibility that the 25 year old male hero of the movie would have a relationship with a 15 year girl. Or think that a 25/16 relationship would be safe.