@lilmagi
Well i don't really get it but these kinds of martial arts "body rebirth" has always been "transforming the user into their prime", when dark was an old man he got younger, so it seems that he did in fact grew older, at least into an age where he can use his body to it's maximum potential, i can barely tell as well since the changes are little since he presumably only transformed two/three years older, but the hair and height did in fact, change.