Cursed people not aging in cursed form.  (Re: Tarou)

This applies in Ranma's case. His curse was age specific; Spring of
Drowned
GIRL. The curse will always give him the body of a girl. Not a woman,
a girl.
There's a difference.

This applies ONLY to Ranma, though: All the other Springs mention
nothing
about age, so the other's forms should grow old at a comparable rate
with
their human counterparts.

I'm pretty sure that Ryoga fell into the spring of drowned piglet, and a
piglet's a young pig right?