I responded to your entire post but I accidentally hit the back button and erased it all. It's been a pretty stressful day. I'm PISSED OFF but I will attempt to remember what I wrote in my previous reply.
You differentiate the physical from the emotional, there is no reason for the distinction, they are effectively the same in practice and have equal importance. Sex as the most physical of human of interactions is concurrently and consequently one its most emotional. By saying one has no effect on the other is plain Cartesian dualism, we cannot be halved between the mental and the sensual.
If by 'emotion' you are referring to physical pleasure, I would agree that there is an 'emotional' component to sex. There is also an 'emotional' component to eating or any other activity that causes pleasure/enjoyment. If by 'emotional' you mean the psychological baggage that most people associate with sex, the onus is on you to prove this and you cannot do so by making an 'appeal to common sense' argument . You cannot prove that other humans are sentient, let alone know what they feel during sex. If there was an inherent, emotional component to sex, we wouldn't be debating this, it would be universally understood in the same way that we universally agree (with the exception of people who cannot feel physical pain) that sawing off one's right arm would be painful.
Sex can be used to express affection or it can result in it but, in and of itself, it is a biological function, just like eating and sleeping are biological functions.
Children are not physically and therefore emotionally developed enough to decide for themselves what is most healthy, thats what childhood is, right? A responsible guardian would use physical force if necessary to stop a toddler sticking a fork into an electrical outlet.
First of all, there's nothing inherently "unhealthy" about sex. Sticking a fork into an electrical outlet can cause physically tangible damage, any distress that having safe, consensual sex might cause a child would come from that child's arbitrary and subjective interpretation of the event. In cultures where child-adult sex is the norm, children don't grow up "traumatized" by having had sex with adults as children because they were raised in a culture that conditions people to believe that child-adult sex is inherently exploitative or that sex is anything more than physically pleasurable activity.
Leaving aside the issue of whether or not we're morally obligated to regard children as our equals (in terms of moral value and worth, not intelligence, maturity, strength etc.) and to respect their self-autonomy, I still think child-adult sex can be justified even from a non-anarchist perspective.
For an adult to enter into such relations with someone who cannot give reasoned consent makes the engagement void and exploitative, one party has the the advantage over another in that they actually know what they are doing and are effectively imposing themselves given that no legitimate consent has been given.
All decisions are 'reasoned', even if a person's better judgement is lacking. Children don't have to be intellectually or emotionlly mature to consent to sex because you've yet to prove that it's anything more than physical contact. If you're going to prevent children from engaging in consensual, safe sex, you might as well prevent them from engaging in masturbation. Although one is more erotic than the other, they both result in the same thing - physical pleasure. You can't decide that consent is illegitimate, either someone fully consents to something without coercision or they don't. Whether you agree with their decision is irrelevant.
Identifying when the "age of reason" occurs is of course the tricky part
The 'age of reason' (at least from a non-anarchist point of view) might apply to drinking, driving, engaging in legal contracts etc. but sex is nothing more than physical contact. It is not inherently harmful or damaging.
indeed there is a broad scientific consensus that males usually only fully mentally mature in the mid-twenties
See above.
Adult will always have "authority" over children; in the sense I described what a child is.
It's not necessarily true that adults will always have authority over children but I've conceded that a loophole in my argument, in practice, is that many children view adults as authority figures and might feel coerced into having sex with them. If the arrangement were fully consensual, I see no reason why it should be considered wrong or inappropriate.
This is not a statist issue any more than gravity is a statist issue, its simply a matter of facts, not idealogies or prejuidices.
Your "facts" are a point of view, they aren't "true". This isn't a statist issue but it is an issue of authority and anarchists are anti-authority/hierarchy. When you violate a child's right to self-autonomy, you are exercising authority over them. Most people aren't full consistent with whatever world views or ideas they hold, anarchists are no different. The same argument you make for parental authority over children can be made by statists for state authority over citizens. Seatbelt laws have probably saved many lives but they're still a violation of self-autonomy. Banning junk food might lower obesity and improve Western health but it would still be a violation of self-autonomy, it would still be an exercise of authority over others.
For the reasons I mentioned, children are not equal to adults in this domain for simple physical circumstances; the adult was born earlier in time. If you think both are at the same emotional level then I assume you're in favour of children (as I defined them) being permitted to care for younger children themselves and even run their own family if they wish?
I've never claimed that children were our equals in terms of intelligence, strength, maturity etc., only that they are our equals in terms of moral value and worth. Because they are our equals in terms of value and worth, they deserve equal consideration and respect, that includes respecting their right to self-autonomy. Allowing children to raise other children is not an issue of self-autonomy because it involves the interests and well-being of other people.
There is such a thing as legitimate authority
No, there is not. Authority might sometimes be beneficial but it is never legitimate because there is no higher, 'objective' authority that can legitimize human authority. Power does not legitimize itself.
a teacher has certain power (in this case intellectual) over his students, but because it is legitimate, it is self-destructive i.e. the teacher's role is to eliminate his power by teaching his/her students everything they know.
A teacher's role is to guide a student's education. That in itself is not authoritarian (their disciplinary role can be), if someone knows something about computers that I don't, we can mutually agree for them to teach me.
By the same token, an adult's/parent's role is to elimate their authority (in this case experience, physical/emotional development) by producing healthy adults.
You don't use authority to eliminate authority anymore than you use hatred to eliminate hatred. I believe the healthiest style of parenting is one that stresses positive reinforcement and compassion and allows for individual freedom and experimentation. A parent's job is to explain to children what the consequences of having unprotected sex, doing drugs, not getting good grades etc. will be, not to punish them or indoctrinate them with their personal values.
I think your framing of this as a "Western" bias towards relationships between adults and children is neither helpful nor indeed accurate to understanding the issue at all. Call me naive perhaps, but I'm not sure many Cambodian parents would be particularly pleased if they knew their child had "consented" to be anally penetrated by some guy next door them.
Most people in the world have similar attitudes about sex.
