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.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.