Leia was born 19 BBY and her ship (with her in it) was captured 0 BBY.
So a clumsy little sum would make her 19, 20 at best, no?
Amidala was born 46 BBY and got bumped by the Trade Federation 33 BBY, so that would have made her 13 or 14 around that time, indeed.

That was either a really stupid attempt from Lucas to make Amidala seem like the early mature type, much like Leia in the OT, or it was just plain stupid, period. At least Leia had reached early adulthood... Amidala probably hadn't even gotten her first...--you know--... :)