How? And please don't note similar physical features. You would be better off comparing him to a non KH character like Zack.
And that's the thing.
Zack is an FF character, whose presence in BBS or KH as a whole is uncertain at this point. Obviously the two aren't connected despite the hairstyles for that very reason.
However, that's not the case with Terra. He's a KH character, and a fairly major one, so it makes sense that they are connected. So the hairstyle is a valid connection. Think about it: in real life, looking so similar to each other can mean absolutely nothing, but in a game like KH, where everything is planned and scripted...
Though that's not really the only way they are connected:
-There's the fact that Xemnas calls Aqua's armor "old friend".
-Xemnas wears knight-like armor in the final battle, similar to the one Terra, Aqua, and Ven wore.
-Terra, Aqua and Ven all have a symbol on their clothing that resembles the heartless insignia. If Terra did indeed become Xehanort in some way, he could have subsconsciously used the symbol as inspiration. Though you might have to consider the fact that it wouldn't be too hard to come up with a symbol like that on your own, read back above to what I said about "coincidences" like this.
-Paralells with Riku, they're supposed to be connected somehow, and from the looks of it, Terra is largely following the same path Riku went down in KH1, dealing with Maleficent, falling to darkness, etc.
-Anger and hatred are both prominent emotions in both LS's and Xehanort's character. Though Nomura said LS may not be Terra, I think it only natural that LS must have SOMETHING to do with him, considering they have the same armor and keyblade, mention Aqua and Ven, hatred for Xehanort, etc. Also, the target of their hatred might be different, but that's not what matters.
There is probably other evidence I have forgotten, but as you can see, there's plenty of evidence to point out that Xehanort is at least connected to Terra somehow, if he's not part of him in any way. It's far more evidence than, say, the one that supports DS being Xehanort.