JuicieJ
SHOW ME YA MOVES!
They could have her be important to the plot but still be born part-ways through, although it would require some unique ideas to get it to work well. I doubt that will happen, though. More likely than not, we'll see a Princess Zelda about Link's age just as we have in the previous games. But actually, I wouldn't be too sure we'll see her as a princess. She could be more or less an ordinary person to start out with, then they bring in the formation of the royal family with her at the head.
Born part-ways through? What, you mean have Zelda born in-game? I mean, I wouldn't call that a bad idea, but when Zelda's part of a game, she's always Link age. (You know what I mean by that.)