As many of you have noticed, young Xehanort and Young Eraqus look the same age in KH III's trailer, yet in Birth by Sleep, Xehanort looks much older than Eraqus, so what is the meaning of this you ask?
Let's start off with Xehanort himself. He is fully immersed into the darkness. Now, think of it this way, he looks the same age as Eraqus in the trailer, but after many years, they became much older, and Xehanort was much older and determined to find a new host body to take over using the power of darkness. Xehanort is much older BECAUSE of the darkness. The darkness makes you age faster, but it only builds up over a long period of time, so that is why Xehanort is older than Eraqus in BbS.
Now since it takes a long time to make a difference in someone's age with darkness, this is why Riku doesn't look older than he should be in KH2. He has only had darkness in him for a few years. So it's natural that he looks the proper age for the time in KH2.
If none of this makes sense to you, ask me any questions you have with this. This is only a theory, so I'm not sure if this is the real reason.
As many of you have noticed, young Xehanort and Young Eraqus look the same age in KH III's trailer, yet in Birth by Sleep, Xehanort looks much older than Eraqus, so what is the meaning of this you ask?
Let's start off with Xehanort himself. He is fully immersed into the darkness. Now, think of it this way, he looks the same age as Eraqus in the trailer, but after many years, they became much older, and Xehanort was much older and determined to find a new host body to take over using the power of darkness. Xehanort is much older BECAUSE of the darkness. The darkness makes you age faster, but it only builds up over a long period of time, so that is why Xehanort is older than Eraqus in BbS.
Now since it takes a long time to make a difference in someone's age with darkness, this is why Riku doesn't look older than he should be in KH2. He has only had darkness in him for a few years. So it's natural that he looks the proper age for the time in KH2.
If none of this makes sense to you, ask me any questions you have with this. This is only a theory, so I'm not sure if this is the real reason.