We know that Eriol is really the same age as Fujitaka, and that he stopped his own aging in order to easily get close to Sakura-tachi. CLAMP never stated whether he would be able to transform to his true, adult self once his mission was completed, did they?
I wonder, how much magic would such a transformation take? Since Eriol's magic was split in half, would he have enough left to perform it? Even after Sakura turned all of the cards into Sakura cards, Eriol is seen still in his childish body, so one wonders why he did not transform already, if he did have the power to. Then, as an adult, he might have had trouble getting on the flight back to England unless he got himself a new passport. ^^
So, what do you think?