At some point River is going to have to be the companion with the Doctor, I mean to know everything she knows about the TARDIS and whatnot, she's going to have to spent a lot of time with him. However, how are they going to show a younger version of her, while the actress is getting older? Eventually they'll have to have a younger River be the companion with the Doctor, he'll meet a younger version of her that doesn't know who he is.