Princess Theory

[Chris211]

It only would have ben 9 years, and they where already adults at the time of BBS. Haven't you ever met someone who looked about the same at 29 as they did at 20.

Nomura stated somewhere you can see a younger version of sora and riku but the PoH retain their age from KH I. From that you can really infer that they were probably at an age where 9/10 years really didnt add any affect to their looks just like AxelsVirtue stated, its as simple as that.