They met when at a dance recital she was a teenager (which I think is kinda strange) but didn't date, and when they did she was 23 and a divorcee. Of all the things he's been accused of so far, this probably isn't the weirdest.
This is what they're now desperately clinging to, to make Moore into a creeper. While different, he did nothing about it till years later.
This is now a 40 year old story give or take, and the anti-Moore taliban are now deperately reaching to try and make him unfit to be Senate, to satisfy their ulterior political agendas. While not aligned with today's morality, it's a desperate grasp to take it across 40 years to try and hook it to this Senate race.