Tbf, while it is state law and so varies, many US states have a provision something like “if over 18, can’t fuck under 18yo, UNLESS you’re within 4yr and have met in an organic way, like school or work.” So an 18yo and a 17yo from the same school would be fine, but a 21yo who met her online or at the mall would still get in trouble for the 17yo, if the parents press charges.