And laws are different for sex work, porn, and anything involving transport or trafficking.
Last I checked it was specifically more illegal in all of the US to purchase sex from some one under the age of majority, regardless if they are over the age of consent for the locality. It carries different charges and more extreme penalties that normal prostituion statutes. Just as pornography or the production there of is illegal if it features people under 18, regardless of age of consent where it was produced or posessed. Or even the legality of child porn where it was made. Transporting a minor (or inducing a minor to transport themselves) across state lines for purposes of sex is illegal regardless of ages of consent in any involved locations.
And those sub 18 ages of consent often come with exceptions/limits of the “within five years age difference” type.
The point of those lower ages of consent is not “it’s a ok to fuck a 16 year old!”. It’s to avoid criminalizing normal sexual behavior between minors and their peers. Nested inside a host of statutes designed to prevent and punish the sexual exploitation of minors by adults.
A 30 year old paying a 16 or 17 year old for sex is an adult sexually exploiting a minor. And it’s rather neatly covered by the host of statutes designed to criminalize this exact situation, without recourse to age of consent. No matter how much “but child brides are legal in Missouri” people want to throw at it.