Imagine that every actor from Until Dawn cosplays as their own characters for example Rami as Josh,Hayden as Sam etc. and they make all together group photos/photoshootings and have a lot of fun together.
So apparently the characters share their birthdays with their actors. And that the game takes place 2nd February 2015, and their characters ages when the game takes place:
Jess, Matt and Ashley -18
Mike, Emily, Chris and Sam -19