Will we be able to send existing YA's to uni?

I don't see why not. The only reason I say it is because I don't have any existing YA's in my game right now, my current family has two adults, four teens & a child. It may also be for some simmers, that their YA's are already established in their careers or have children, so sending them to uni may not be conducive, or they're just looking forward to this new step in the transition from teens to adulthood, rather than just jumping right into their careers. But there really should be no reason why existing YA's or even adults & elders can't enroll, like everything else in the game, it's all down to player preference & how the new content fits their narrative.