I think it's the high levels of cultivation that allow Yu Yin and his disciple siblings to remain young looking. And I'm guessing that the whole body thing is either related to him being called an immortal, or some other plot hijinx that will be revealed
<3<3<3 the princesses finally meet again! <3<3<3
I started reading the novel because I couldn't wait to know what happens next, but it only made me more obsessed with this story, and I want to just reread it over and over again