We've seen that Aqua and Even have not aged. So, it wouldn't be right if Terra just did. It's also kinda weird to think about, when the three are back, they'll be around the same age as Sora and the others. But technically they're over 10 years older. ' I wonder if Aqua will still want to pat Sora and Riku's head?' Even though Riku will almost be the same age as her.
Aqua might end up with Riku if Terra has a tragic end.. just hoping because Riku needs someone..