Actually, I alluded to what happened to Jenna way back in the beginning of Reborn. It was after Jess and Nate's 'first time' in the boathouse at Anna's party, and Jenna was furious and the things she said led Holly to ask if something had happened to her, to which she responded firmly that she was Not Talking about it. And then when she's thinking just after that I alluded to it some more.
As for Harri, there's no real fair comparison to the girls, remember she's Jenna's Great Aunt, her bloodline keeps her younger but she's actually in her sixties, while they haven't even turned twenty yet :)

I'm so glad that you are invested in the characters, that is what I care about most! Your comments mean the absolute world to me my friend!!