If the question at hand is, “How would we determine the legal matter?” I will elaborate on that in response to the OP. But first some groundwork must be laid.
The first response brings with it an interesting commentary:
This is precisely the line of thought I have a problem with. “Informed decisions” supposes an explicit or implicit standard about the individual in question, specifically whether one is “informed”. The problem here, I think, is that, how can one be sure of the consequences of sex without engaging in that activity? (I suspect that anyone who states this is thinking of “pregnancy” or “sexually transmitted diseases”, but not “trust”, “intimacy”, and possibly abuses thereof which, I think, are more characteristic of social sexual behavior than the others.) Before anyone quotes that question, let me elaborate my concern.
In general, instruction on any matter takes a few charactistic forms. One is the telling of the situation: ostensive definitions, exemplars, an attempt at a demonstration of the pure article by means of hard-and-fast causal links or pictures or etc. The second is the practice of what was just told. For instance, in learning mathematics, one does mathematical problems. This is putting the ostensive definition to use in order to give the instructor (which may be a person or a peer group or…) the ability to judge whether or not the person understands. I will remark here that there is no hard and fast rule which corresponds to understanding in the instructed.
For behaviors humans engage in, especially social behaviors, the practice of it is generally much more important than the ostensive definitions. We generally do not think of much behavior as being so defined (sharing, for example), though such definitions are possible.
The question at hand, then, is what indeed would count for an “informed decision” about a behavior which, for practical as well as legal as well as cultural reasons, cannot be taught by practice but only be ostensive definition (health class; “when a man and a woman love each other very much…”; etc.). I would suggest that one can know a great deal about the operation of sex through ostensive definition, but that this will never, ever amount to understanding of the social behavior such that one can make “an informed decision.” The test of “doing something right” such as a social behavior is in doing it, and being corrected as necessary. (Sharing, speaking, skipping, building, explaining…)
I do not mean to nitpick caphis as such (I do not wish to attribute any position to the poster, only to take that sentence as an example of a thought I find wrong. I do not know if the poster feels the way I read it, and I do not wish to accuse.) I mean to call into question the heart of the matter of “legally qualifying to engage in a legal behavior.” I mean to suggest that no rule will do here whatsoever because we do not teach, and by necessity do not learn, social behaviors strictly via ostensive definitions. Without practice and correction the ‘correct’ behavior will not be learned.
This presents us with a difficult situation with respect to the sexual act specifically. There is a strong cultural selection process which generally forbids sexual activity with minors by adults, and an even stronger selection against those adults being family or teachers. But the human animal is nothing else if not an explorer when it is young, and it will recognize, implicitly, a gap in instruction, and for several reasons will end up wondering about sex. Because the sexual act is absolutely necessary for human survival, it is hardly something we can actually (or practically) shield them from.
With that groundwork laid, I can procede to tackle the OP and say that teenagers probably don’t have a civil right to have sex, but that we should not go out of our way to punish them for wanting to learn about a necessary behavior we simply refuse to instruct them on. I say this because we generally wish to shield children from some of the “adult” consequences of their actions so that they have a safe environment in which to learn how to be, and if they are not relatively free to make mistakes, we can hardly instruct them in any meaningful way (i.e. we would require that they understand what not to do before we tell them!). We should try to offer them similar protections, it is true, but I do not believe it is appropriate to automatically transfer any particular civil right to children just because they happen to be citizens.
Forget liberty. Sex is necessary. If humans do nothing else with their lives, eating and fucking are paramount to the survival of the species. This is a behavior which should be protected before all others. This is not to say it should be encouraged, but that discouraging it is a sure path to problems of so many kinds I dare not even contemplate it.
Sexual activity goes beyond simple reproduction. It is also the foundation, generally (we must always speak of general cases here and ignore the outlying exceptions which do not serve to characterize the species), of healthy relationships by fostering intimacy, which fosters (hopefully some level of pre-existing) trust, which is the bedrock of social behavior in general. Whether I have sex or not, if there is no one I could possibly trust to have sex with, I would humbly suggest that I could not meaningfully participate in this society. (Note I do not require that anyone actually exist that I desire, and I do not mean to suggest that I must have any libido whatsover, only that the level of trust required to have a “healthy” level of sexual activity corresponds to a great many social behaviors in general.)
Generally that the costs are too high. Teenage pregnancy is not a particularly good way to enter into this society, not for the parent or parents, and not for the child. But the question at stake here is whether we can meaningfully forbid them from having sex while at the same time establishing the phenomenon of trust, responsibility, and privacy… all of which lead indirectly to sexual behavior (specifically the kind of sexual behavior we approve of). Anyone can write a law. But can it be enforced? --And what is the cost of enforcement?
This question is probably outside the scope of what I can give a concise answer to. I have problems with the perception that by virtue of being an adult one is in a position of authority over any arbitrary child. If that is so, it is more because of the fear and respect we instill in children than it is anything special about the relationship. But the flipside is clear: rapid instruction requires such a relationship in place. The question then becomes one of whether or not it is appropriate for the state to restrict relationships based on authority roles in general, or whether only specific roles lend themselves to abuse (and I do not mean sexual abuse, I mean “a corrupt practice or custom” to borrow M-W’s definition 1). That is, to take it as a given that such roles exist, is it necessary that all of them distort the practice of privacy, trust, and, in some cases, instruction? (Note that not only the young are instructed.) I would answer that, yes, probably, there are some instances of authority roles which, when coupled with what we characterize as a healthy relationship (friendship, business partner, lawyer, romantic partner, etc.), lend themselves to practices which distort their ostensive purposes.
It is fair to close with the remark that “being instructed in sexual activity” is not a process which has a definite end, any more than “understanding” any activity that must be practiced to be appreciated (which is generally all social behaviors). This means that an arbitrary age cut-off only makes sense under certain conditions, the constitution of which can be derived, I believe, from the previous paragraph.