On Fri, 6 Dec 1996, Sebastian Weinberg wrote:

Um, well, this is probably going to become a major point, so I am
uncomfortable about saying this, but.... the girl part exists only
in the english translation, and the adjective "young" doesn't come
into it at all.  I am not 100% sure whether the "Nyan" part means
woman or girl, but judging from the kanji used for it, I would say
not.  The kanji used in the Manga just means female/woman.

He could always use an alternate explanation:

Ranma's forms age separately and only while he's in those forms.  This
means if he ages 20 years and is male half the time and female half the
time, his male and female forms each only have aged 10 years.

This even gives a much better explanation as to why _male_ Ranma is also
young as well as female Ranma.  You don't need funny explanations about
nobody really understanding how the curses work.

(And in either case, if people who fall in the girl pool don't age much,
that's a hell of a reason why you might _want_ your children thrown in
the pools....)