Terra (in age) is probably anywhere from 18 to 25. Why such a large gap? Simple. Terra looks a different age in just about every single shot I've seen so far.
According to what we've read on the Birth by Sleep trailer (and to what I understand and interpert), Terra is on a beach at one point and we can see (at least, it
looks like) Sora and Riku sword fighting in the distance.
Click here for scan with picture. Yeah, I say it
definately is Sora and Riku. Nomura did say that the furthest back Birth by Sleep can be is about ten years. That would make Sora four and Riku five anyways, wouldn't it?
Now, let's do some math. If Terra is 18 in Birth by Sleep and Sora is four, there'd be a fourteen year gap between them. It's not
impossible to imagine that Sora and Terra are siblings. Terra would've been fourteen when Sora is born. My friend is the middle child of three and her older brother and younger sister are sixteen years apart.
If Terra is 25 and Sora is four, than there's a twenty-one year gap between them and because Terra would've been 21 when Sora was born, it's not inconcievable that Terra could be Sora's father in this case. It's just unlikely that Terra would have had time for a serious relationship such a marriage if he's an apprentice.
Now, looking at Kingdom Hearts; their heros have started out young, fourteen and fifteen. I think it's more likely that Terra's about eighteen. Nineteen or twenty at the most perhaps. But both of those ages leave a gap that make it more likely that Terra and Sora are siblings.
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To anyone who says a strong heart is inherited thru blood.... I laugh at thee. Genetics makes up personality only in science and it's some of the most ridiculous stuff I've ever read or heard about. It's relationships and the things that have happened to you that make you who you are.
