In Fruits Basket, different characters represent different zodiac symbols that they then turn into when they are hugged by someone of the opposite gender. What determines what zodiac a person represents? Is it the year they are born in? If so, what determines who becomes the cat? Additionally, does that mean that every cursed Sohma is born in a different year (discounting Akito)?
It is never explained what makes some Sohma born with the curse and some not and what animal they represent. The only rules seem to be that only one person can have an animal's the curse at a time and they must be part of the Sohma clan. The current condition where all animals are represented at the same time is rare.
Although some of the years do potentially match up (Ex. Shigure can be year of the dog 1982 and Tohru can be year of the dog as well in 1994, Ox is a year younger than the Rat, the Snake and the Dragon are close together), the year of birth isn't the single determining factor, since the ages don't match for all the zodiac members. (Ex. Momiji is the same age as Hatsuharu and slightly older than Kisa.)