You can find a transcript from George Lucas, Steven Spielberg, and Larry Kasdan's original brainstorming sessions for this film online (http://maddogmovies.com/almost/scripts/raidersstoryconferenc...). In the transcript it's made clear that she would have been underage, but in the film it's only hinted at. Here's an excerpt:

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...