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.
It's kind of funny. They confirmed in the last episode of LoK that Bumi is the oldest and yet Kya has completely grey hair and Bumi doesn't have a single grey hair. I guess it's hit attitude or something keeping his body young.