Stumpy's adult self is more mature than his kid self, even though Mr. Cat explains that he only aged physically and not mentally. However, adults have different neural networks and hormone balances than young children, which can also affect one's personality.
Stumpy turns into an adult because of magic and decides to leave Smileyland. A screaming, wailing Kaeloo runs to Mr. Cat for comfort and cries to him about how she doesn't want Stumpy to leave. Mr. Cat, who until this point seemed proud of Stumpy for finally growing up, immediately changes his mind and uses the same magic to make Stumpy so old that he can't go anywhere without them. Kaeloo is overjoyed and grateful to Mr. Cat. In other words:
Mr. Cat is perfectly fine with incapacitating his good friend just to keep Kaeloo happy.
Kaeloo is fine with making Stumpy dependent on the main four so he won't abandon them.
