2 Augmented the answer to clarify that some of the zodiac members DO match up, but the original answer stands that it's not the only determining factor and what determines the zodiac members isn't clearly defined.
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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.

TheAlthough some of the years do potentially match up (Ex. Shigure can be year of birth can'tthe 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.)

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.

The year of birth can't be a factor, since the ages don't match. (Ex. Momiji is the same age as Hatsuharu and slightly older than Kisa.)

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.)

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source | link

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.

The year of birth can't be a factor, since the ages don't match. (Ex. Momiji is the same age as Hatsuharu and slightly older than Kisa.)