With regard to the Delphi killer's victim selection, I think it's possible he has previously been to the bridge/trail area or maybe just wondered/fantasized about the potential for taking a victim at that location. Then on the day he's selected to try to make his fantasy happen, he's been out there awhile. Maybe he figured he'd come upon a woman walking alone earlier in the day, but the circumstances just didn't come together right, as even on a nice day it's not a real well-traveled trail. By the time he sees Abby and Libby, he's already figured out that 1. There's not a lot of opportunities for a single victim, and 2. The end of the bridge is a good place to confront a victim. So when he sees them, he knows he's going to have to improvise from his original plan. He can't be picky about appearance, age, or even the fact that there are two of them if he wants to make it happen today.
So possibly their exact ages and the fact they were a duo didn't matter so much as them being female and ending up exactly where he wanted them location-wise.
Just speculating.