When Jim finally gets to ride the merry-go-round forward to become older, Will manages to pull him off, but not before Jim has made at least one revolution of the merry-go-round. What bugs me is that this fact is never addressed. Will knows that Jim has aged at least one year, so why doesn't he bring it up after he and his father save Jim? Granted, it isn't that big of an age increase, but Jim could have ridden the merry-go-round backwards to erase it before Charles destroyed it (even if the merry-go-round really couldn't work with the freaks gone, they could have at least tried). Is there a reason for this other than "the author forgot or decided that resolving it would make the ending unnecessarily long"?
Jim was the one in a hurry to grow up. It's possible he would have refused to ride it backwards anyway.
I remember thinking this was sort of like a price he paid for trying to use the merry-go-round...by pulling him off they kept the aging to a minimum, but the book ends with him still irreversibly effected in a small way.
There's also the characters' realization that if they use the merry-go-round, even just a little bit, they'll never be able to destroy it. They'll eventually be seduced by its power and become what they just defeated.