Final answer:
While the original question asks specifically about the percentage of adolescents who have had sex before the age of 20, the information provided focuses on the attitudes and beliefs towards sex among adolescents rather than explicit percentages. Studies indicate shifting attitudes towards premarital sex and childbearing outside marriage among American youth, as well as a desire for monogamy in the future.
Step-by-step explanation:
The question pertains to the prevalence of sexual activity among adolescents before the age of 20. According to various surveys and studies, there has been a shift in the sexual attitudes and beliefs among youth. While direct statistics for sexual activity before the age of 20 aren't provided here, available data suggest shifts in attitudes towards premarital sex, the acceptance of out-of-wedlock childbirth, and the ideal of monogamy.
The differing sexual attitudes can be gleaned from the fact that a majority of both adolescent boys and girls seem to accept the notion of having children outside of marriage, and teens acknowledge the challenge of abstaining from sex until marriage, but this does not preclude a desire for a single sexual partner in the future. However, this data should not be directly conflated with the percentage of adolescents who have had sex before 20, as the question originally asked.
The context and the availability of sex education play an important role in forming these attitudes, especially in the United States, where there's a variety of opinions on the matter. In contrast, the sexual openness of other countries, compared to the relatively conservative stance of the United States, indicates a broad range of norms and values pertaining to sexuality.