LAWS DEFENDING TEENAGE RELATIONSHIPS (Age Gaps)

Age differential.
A number of counties/ states specify age ranges outside of which parties cannot consent to sex. In State B, sex with an individual under 16 years of age is illegal if the other party is four or more years older. Thus, sexual relations between a 15-year-old and an 18-year-old would be legal, while the same relationship between a 15-year-old and a 21-year old would not.
(Reference: https://aspe.hhs.gov/execsum/statutory-rape-guide-state-laws-and-reporting-requirements)

ROMEO AND JULIET LAWS (USA)
Under Texas' version of the law, if a young adult over the age of 17 has consensual sexual relations with someone under the age of 17, but at least 15 years old, with no more than a four-year age difference between the two, the new law will not require the older party to register as a sex offender if convicted of statutory rape.
(Reference: http://www.theshapirolawfirm.com/News-and-Resources/Articles/Romeo-and-Juliet-Law-in-Texas.shtml)