In contrast to Simeon ben Yohai's dictum that sex with a little girl is permitted under the age of three years, the general teaching of the Talmud is that the rabbi must wait until a day after her third birthday. She could be taken in marriage simply by the act of rape.
R. Joseph said: Come and hear! A maiden aged three years and a day may be acquired in marriage by coition and if her deceased husband’s brother cohabits with her, she becomes his. (Sanh. 55b)
A girl who is three years of age and one day may be betrothed by cohabitation. . . .(Yeb. 57b)
A maiden aged three years and a day may be acquired in marriage by coition, and if her deceased husband’s brother cohabited with her she becomes his. (Sanh. 69a, 69b, also discussed in Yeb. 60b)
It was taught: R. Simeon b. Yohai stated: A proselyte who is under the age of three years and one day is permitted to marry a priest, for it is said, But all the women children that have not known man by lying with him, keep alive for yourselves, and Phineas (who was priest, the footnote says) surely was with them. (Yeb. 60b)
[The Talmud says such three year and a day old girls are] . . . fit for cohabitation. . . But all women children, that have not known man by lying with him, it must be concluded that Scripture speaks of one who is fit for cohabitation. (Footnote to Yeb. 60b)
The example of Phineas, a priest, himself marrying an underage virgin of three years is considered by the Talmud as proof that such infants are "fit for cohabitation."
The Talmud teaches that an adult woman’s molestation of a nine year old boy is "not a sexual act" and cannot "throw guilt" upon her because the little boy is not truly a "man.” 9 But they use opposite logic to sanction rape of little girls aged three years and one day: Such infants they count as “women," sexually mature and fully responsible to comply with the requirements of marriage.
The Talmud footnotes 3 and 4 to Sanhedrin 55a clearly tell us when the rabbis considered a boy and girl sexually mature and thus ready for marriage. "At nine years a male attains sexual matureness… The sexual matureness of woman is reached at the age of three."