I don't think you're wrong-- the manga shows it's age, especially in that scene where Yashiro insinuates girls grow up awfully fast... Always gives me the heebie jeebies.
Though, I think it's a little more palatable when you consider Ren only fell for Kyoko once he realized she was the girl from his childhood-- Where their age gap was a lot less "predatory"... Though, I can't help but wish Kyoko could've been aged up just a few years...
It is creepy. Once both are over 18 (20 is better) it really doesn't make any difference - it's only a 4 year difference, but younger than that it's not good. It's worse when you think that Sho was sleeping with his manager (26) and that director lady (older than 26, I think), and he was 16. That makes me want to vomit. Yashiro isn't wrong in that girls mature faster, but he wasn't meaning it in a predatory way - he meant that mentally we grow up faster and are able to handle relationships better at that kind of age... he also said not to do anything illegal.
Everyone talking about the manga quality and length but I'm here thinking about how REN a 20/21 YEAR OLD MAN fell in love with a 16/17 YEAR OLD GIRL. I started reading this manga when I was a pre-teen and I thought this was normal but now as someone closer to Rens age I can't fathom romantically liking someone that young. Literally so weird that an adult came up with this concept as well