I think you’re stuck on the legal definition of an adult.

Some countries have people become adults at 16, some at 18 and some at 21. 18 is meaningless beyond the laws in place and the legal punishments given to those who break the laws.

Many other reprehensible acts are legal, such as firing people for their ethnicity or gender identity in the US, or running a payday loan scheme.

You need to look at it from the perspective of brain development and power dynamics rather than from a legal or subjective point of view.

Intercourse with someone below your level of mental maturity is morally reprehensible due to the mental harm it may cause to the younger person, and them being unable to consent properly due to the power dynamics of the older partner having more money, more maturity, more status, etc. The younger individual may become dependent on the more mature individual, further driving the negative power dynamic.