As mentioned by
Dr.Fapalot, it's brought up during the first Ellie shower. And then again when Sam invites Sophia to the concert, when she asks if he finds Ellie attractive as well (who doesn't?). Then in the day 19 shower Sophia reminds Ellie, who sounds enthusiastic. And finally last update, but we learn that Ellie might have something with friends. Hence my theory she might just be running late. I mean, if I was her, I'd much rather be playing around with Jules' magic tongue.
Which (at least to me), sounds like L&P is setting Sam up as one of the three Ellie branches - Logan and precious little Jules being the others.
Since this is the last time Sam and Ellie are likely to be in the same place before the Ellie branches are slated to begin on playable day 16. So the last time Ellie really has in order to have any reason to break her rule about dating younger guys. Sam being a performer (like her) and having his own car (even if it was just given to him), might just be enough to let her bend the her rules.
And, since L&P appears to like using other characters as coitus interruptus as a means of keeping up suspence, Ellie is perfectly suited to the task. Although since Sophia and Patricia appear to have chosen the rear of the tables and are keeping their distance, I still think that it's the playable day 13 event in which Sam and Sophia might get some alone time.