I think what is missing from this whole debate about puberty and it's consent making properties is brain development and emotional maturity. Which, whilst tied to puberty, does not necessarily develop at the same time.
In other words a 15 year old might be highly intelligent and might even be properly physically developed. That does not necessarily mean they are emotionally mature. It is this emotional state of being that is thought to be the reason sexual encounters might result in detrimental effects on the teen/child.
Studies of the teenage brain (which is a relatively new focus) show that the teenage brain is actually unique. It is not, as previously thought, simply a young adult brain, neither is it a child's brain. According to researchers it is actually at a crossroads. At the end of childhood but at the same time not yet an adult brain..
Whilst this level of development may be beneficial (better learning retention skills to that of an adult brain) there might also be inherent vulnerabilities in the teenage brain compared to that of an average adult brain.
The differences are that an average teenager is (surprise surprise) usually more moody, more prone to risk taking behavior and more prone to react emotionally rather than logically or rationally. This is because of the way the brain develops.
Because the brain develops from the back first, the last thing to properly develop in adolescents is the Frontal Lobe or Cortex. This is actually part of the brain which an adult uses to properly asses danger and decide whether the risk is worth it or not. Because this is actually immature in teenagers (up until I think 15 -17 depending on the individual, could be wrong on those ages so don't quote me on that) the risks a teenager takes with regards to the usual teen antics (sex, drugs, alcohol, smoking etc) is usually seen as not as informed or properly dealt with, compared to an adult taking the same risk.
Because the last thing to develop is the Frontal Lobe, teenagers actually process the emotions of others with the
Amygdala instead. This gives them inaccurate information and as such young teenagers are usually not so great in discerning the mood of people around them. It is this reliance on Amygdala instead of the Frontal Lobe that gives teenagers that stereotypically moodiness we have all come to expect.
It has even been shown that because the brain is still developing during adolescence that what a teenager does lifestyle wise, might translate into the way their brain develops.
With regards to teens getting it on with other teens, this is normally seen as little more than experimentation and reacting strongly specifically to Biology rather than combining it with rational responses like a sober adult usually does (some teens might be rational about it, of course.) As such even in cases pertaining to statutory rape between two teenagers, even though technically against the law, there might be a certain leniency given to the right circumstances. That leniency is not afforded to an adult (age wise, depending on the country) having sex with a teenager, even if they are of consenting age. For example in Australia 16 is the legal age for consent, but anyone over the age of 25 (and anyone 18 and over if they hold a position of authority like guardian or teacher) having sex with even a 16 year old will often be charged. This is because an adult is seen as more culpable and better able to assess their behavior than that of a teenager, even if they are of consenting age. Coupled with the fact that we know teenagers are usually more prone to emotional responses than rational ones (generalizing here, of course) the adult is seen as someone who ought to know better.
