Because they don't, he said as much in the Days Ultimania.
Plain and simple, the timing was retconned, and has been since KH2 (given what AtW says in the SAR).
Maleficent didn't take over Radiant Garden a year after BbS, but in fact several years (I'm actually leaning on closer to KH1 than BbS). Meaning, by extension, that there was room for the apprentices to age, which they clearly did.
And... scratch that.
We just got confirmation that Nomura still says it was nine years ago. So, guess what?
+1 retcon: Was originally 9 years ago in KH1, less in KH2, and back to nine again.
+1 plothole: There is no conceivable way that Ienzo in particular aged that much in one year, he must have aged as a Nobody even though Nomura explicitly said Nobodies don't age. The effect of age can also be seen in Xemnas and Xigbar.
Lovely. Let the plotholes keep piling up!