When can an individual be considered a moral agent?

Obviously, certain individuals in society lack the mental capability to consent to certain interactions with other, e.g. babies, comatose people, people on drugs, etc. As such, the responsibility for making some or all of their life decisions must be temporarily or permanently transferred to another individual.

Fundamentally, people are entitled to natural rights if they’re capable of fulfilling the duties associated with those rights. Under the NAP, this simply means respecting the life, liberty, and property of others. As such, people have the right to just about anything they want to themselves so long as they respect the identical right of others to do so. The only limiting factor here is their ability to mentally desire things and voluntarily acquire them. For actions which an individual does purely by themselves for themselves, this unambiguously applies. For example, growing marijuana plants or taking nudes of themselves. However, issues arise when you consider interactions with others. Individuals are equally self-interested and have an incentive to manipulate others to do what they want. Obviously, things like scamming violate the NAP, so it stands to reason that people could be ethically and epistemologically manipulated in other ways that constitute a NAP violation.

However, I’ve never seen a good answer as to where the bar should be set with this. At the same time, applying rules arbitrarily without a rationalist backing which can be applied to all individuals is going to lead to socially inefficient outcomes. As an example, one might feel that individuals under the age of 18 cannot consent to sex with those over the age of 18. However, the chronological age of these individuals itself is not relevant to their ability to consent to sex; their mental development is. As such, it stands to reason that other individuals who lack mental development should not be able to consent to sex either. You then run into all kinds of other questions as to who can consent to sex. For one, why should this rule be applied to sex and not, say, riding in a car? The latter is far more likely to kill you, statistically speaking, and thus constitutes a much more significant mind-independent risk for one to consent to. If the issue is whether or not one can understand the relevent costs and benefits of sex, then that applies to virtually any interaction you can think of. Virtually no one knows the intricate details of every decision they make on a daily basis, such as consenting to medical procedures. Basing a rule off of this is going to force you to make arbitrary assumptions. What makes 18 not arbitrary? Why not 19 or 17? Should a universal rule for consent be based off IQ? If so, what score should that be; 60, 70, 80? What makes that rule not arbitrary? Should minors be allowed to have sex with each other, but not adults? You could argue the difference in mental capacity is what matters here, but you run into issues there too. Should it be illegal for someone with a college degree to have sex with a high school dropout? Should it be illegal for someone who got 8 hours of sleep last night to have sex with someone who had 2 hours of sleep? These are issues that have to be resolved if you’re to have ethically-consistent legislation on who is and isn’t a moral agent.

The real issue isn’t whether or not certain individuals can or can’t consent to certain interactions with others. They clearly can; even babies can express that they don’t want something by crying. The issue here is one of who’s mentally capable enough for their consent to be valid. The best benchmark I’ve been able to come up with here is that people demonstrate that this is the case by being capable of seeking recourse for violations of that consent. For example, someone who’s raped can call the police or sue their rapist if they feel the sex was nonconsenual. For an individual to be capable to consent with another for any given interaction, then in addition to the aforementioned need to fulfill moral duties, they also must have the mental capacity to express nonconsent and concurrently be able to seek resource on their own for violations of that consent. To me, this rule can be effectively applied to essentially all questionable cases of moral agency, and while it isn’t perfect, it’s the best I’ve been able to think of. Let me know your thoughts on this though and if you can think of something better.