The reason it's so strange has to do with the rapid aging of the King. I changed a lot when I was growing up through my teens, now that I'm 25 I haven't really changed how I looked over the course of the past few years. Cor is 42 years old, and yet looks exactly like how he does when he was younger when Noctis was little and Regis didn't look old. The reason it seems like they aren't aging is because the gap in time isn't really as large as you would think. Noctis was 8 in Platinum Demo, which would mean that it's been only 12 years since he was that young. When you consider in the anime Noctis was older than 8 as he had recovered by then and looks a few years older, you're looking at at least only 8-10 years difference. Not really that much to change your appearance, especially if you've already matured like Gentiana and Cor have.