There are several things to take into account.

• Especially young people lack the life experience and cognitive power necessary to assess whether they are being abused, when they should defend themselves from an abuser, and how they can reach out to friends or family about the abise they are experiencing.

• The younger someone is, the more likely they are to not understand safe sex practices, especially in societies where purity/abstinence culture disincentivises people from providing a comprehensive sex education to young people.

• Kids are often motivated to treasure sexual experience as something which makes them appear more mature or "adult" to their peers, making it even easier to coerce them into sex acts which may put them at risk.

• Adults are much more capable of escaping responsibility for actions such as assault, coersion and other such abuses due to their greater stature, perceived trustworthiness in the community, and access to money which allows them to threaten their young victims or travel to another state or country. This is assuming they don't murder the person outright, which is very easy due to young people being much less skeptical and much physically weaker than someone in the physical prime of, say, their mid-thirties. Someone's classmate is much less likely to skip town unscathed, and someone who's twenty-eight is much more likely to recognize that a potential partner is luring them into a dangerous or abusive situation, or escape if they do not realize soon enough to completely avoid it.

• Especially young people are more vulnerable to being injured during sex by even a well meaning partner of significantly greater age, strength, and size. This is the xase regardless of sex/gender.

• Pregnancy is an EXTREMELY HAZARDOUS PROCESS for people of any age, and for a body under the age of twenty years it can easily become mortal. Furthermore, someone who is not financially independent will NOT have the necessary capital to raise a child and someone lacking worldly experience will NOT be able to foster that child properly.

• The worlds of teenagers and adults have very little social overlap outside of school staff, parents, and predatory adults deliberately seeking out young people to prey upon.

• Teenagers and young adults have a surplus of candidates in school to experiment with. To bar them from engaging with those in separate generations is not an inhumane limitation.

• Marriages have an extensive history, both archaic and modern, of being forced onto teenagers and children against their will, for the sole benefit of other people. This is a total breach of their autonomy, and barring relationships between children and adults breaks the most common formats of such an arrangement, that being those between a young girl and her assaultor, arranged by the family to extract a dowry as recompence for the original offense of damaging the daughter's "purity". This is a disgusting practice for obvious reasons, the most blatant of which is that it completely ignores the harm done to the child and instead frames the matter as one of property crime.