Well Kya was intended to be the oldest at first. For Bumi to suddenly be the oldest doesn't seem to fit in my opinion - I mean, Kya's hair is already gray for crying out loud! Bumi's hair is still brown and even we can tell Tenzin is not in the gray age with his eyebrows.
I think Kya's hair is white, not grey. If you look at a picture of Aang's family when the kids are still young, you'll see that Kya's hair is white THEN, too. So hair color isn't exactly a good indicator of Kya's age.
Also, how do you know that Kya was intended to be the oldest? Do you have a source of information?
It's been a while but I remember Kya being the oldest was part of earlier conception. And what do you mean her hair was while as a child? The photo maybe black and white but the colors are still obvious. If her hair was naturally white, she would've been related to Yue somehow.