In most games, age is hardly a basis to prove or disprove theories, since it's usually something that's pretty vague, changeable, or easily retconnable. Sure, Nomura has stated Xehanort was 30. Even then, he said Xehanort "seems" 30. This would imply that that was the general age that Nomura had in mind, and when confronted with the question, that's how he answered. Nothing more. This is especially uninmportant with Terra/Xehanort, since they do look remarkably similar. I guarantee Nomura would have no problem retconning his previous statement since they already look so similar anyway. They're anime esque characters with no evidence whatsoever of aging and perfect faces. Age hardly matters in this case unless they're notably young like Roxas, or notably old like MX. Besides, have they confirmed Terra's age anywhere? I haven't been up to date at all with BBS speculation, so I'm not sure. But in my opinion, Terra looks 22-25, Aqua 18-21, and Ven 15-17. If Terra's in his early/mid 20s, that's not nearly different enough to theorize that it won't be retconned.
In any case, I can't see why everyone is so obsessed with trying to disprove Terra being Xehanort. In my opinion, it's just painfully obvious. It's like they've been TRYING to tell us in the trailers, and people just think it can't be so. Is there a chance that there's more than Terra being purely Xehanort? Of course. In my opinion, Vanitas or MX will be very involved in how it ends up happening. That, or the things that have happened to Vanitas will connect to Terra's transformation. Or, his turning into Xehanort will certainly be resultant of the Heartless/Nobody's creation. Who knows. But dear lord, can it be any more obvious that Terra is supposed to become Xehanort?
-They look EXTREMELY similar. Hair, facial, and body structure.
-He gives into the darkness, etc.
-His eyes turn yellow, as if to tell us blatantly that it's hinting at Ansem's heartless.
-Xemnas calls Aqua's armor "friend."
-The story so far that we've seen has been heavily emphasized on Terra, hinting that the events happening to him are the most important of the story, and along with that, the series.
-Many games have established prequels to highlight one of the major characters of the series. Example, Metal Gear Solid 3 left a heavily unfinished story hanging to establish Big Boss' character and origins. It's just obvious they're doing the same here.
I don't know. Am I the only one thinking this? There's more evidence and theories I could post, but that's probably for another thread.