Isaac was forty years old when he married Rebekah, sixty years old when Esau and Jacob were born, while Abraham was 160 years old. He and his twin brother, Esau, were markedly different in appearance and behavior. Esau was a ruddy hunter, while Jacob was a gentle man who "dwelled in tents," interpreted by most biblical scholars as a mark of his studiousness in the "tents" of Torah.
And the boys grew: and Esau was a cunning hunter, a man of the field; and Jacob [was] a plain man, dwelling in tents. –Gen 25:27
During Rebekah's pregnancy, "the children struggled together within her" (Genesis 25:22).
According to Rashi
Whenever Rebecca passed a house of study, Jacob would become agitated, indicating his natural inclination for Torah study. If she passed a house of idol worship, however, Esau would become agitated, expressing his own penchant for idolatry. (Rashi, Bereishis 25:22) Seeking to understand this paradox, Rebecca approached Shem, the son of Noah, who had an academy of higher learning and was also a prophet. He revealed that Rebecca would give birth to twins whose character would be completely divergent from each other. Jacob would embrace righteousness; Esav, his evil inclination. (Rashi, Bereishis 25:23)More...