There is a good post over at Say Anything called "Predatory Sex".
For most of the existence of homo sapiens, covering about 130,000 years or more, the typical lifespan was about 20 years. During the overwhelming majority of our existence, I think it is safe to say that boys and girls had sex at, what we would consider, very young ages. It was necessary for the propagation of our species.
In roughly the last 30-40 years of our time on this planet, Western Civilization has determined that this is wrong. This was a necessary determination due to MANY factors, which I will not go into here. Suffice it to say, our culture has evolved faster than we have.
That is not to in any way excuse pedophilia. But I do think we need to differentiate between consensual and non-consensual acts for the purpose of determining appropriate punishment. Obviously, non-consensual acts should be given the harshest punishments. The involvement of a child who is too young to understand sex obviously falls into the non-consensual category, since they are incapable of understanding to what they are consenting.
Unfortunately, our society likes to draw hard lines. At age 18 (in some states 16), you are suddenly a sexually mature and functioning adult. Sorry, but that is complete bull (otherwise, why is the drinking age 21?). Some people don't become mature and functioning adults until 25, or 30 (some people never do). Other people could handle it at 13. We basically have an arbitrary line drawn which we apply to everyone only because we are unable to measure maturity.
But let's be realistic about this: When a 30 year old woman has consensual sex with a 15 year old boy, who is hurt by this? Maybe I'm missing it, but I don't see the victim here. When a 15 year old boy has consensual sex with a 15 year old girl, we may frown on it, but we don't send either of them to jail.
I should define consensual sex. To me, it means both people could say no, but both people choose not to say no. In anyone's life, there is a point where they would understand WHY they would want to say yes or no. Sex prior to that point should be forbidden. There is your true age of consent.