"Rachel and Leah were twin sisters, fourteen years old when Jacob came to their father's house; consequently they were twenty-one years old at the time of their marriage to Jacob (Seder 'Olam Rabbah ii.).
The terms "elder" and "younger," applied respectively to Leah and Rachel (Gen. xxix. 16), are explained by the Rabbis as referring to the divine gifts bestowed upon their descendants; for while royalty and the priesthood remained permanently with Leah's descendants, they were held only temporarily by Rachel's—royalty with Joseph and Saul, and the priesthood with the tabernacle of Shiloh (Gen. R. lxx. 15).
In other respects the two sisters were alike, both being ancestresses of kings, heroes, prophets, judges, and conquerors."
(Jewish Encyclopedia pg.305)