Quite, jake, but, as you�re always telling us, things change and society has moved on a little in the last thousand years.
Children are generally not able to make reasoned decisions about many things. For example, whether or not they should smoke, or drink, or drive or have sex. Yes, they are all different and mature at different times in their lives (and some, especially men, perhaps never do so at all). Generally their parents ensure they behave in a way that does not jeopardise them or others around them. However, some parents do not.
The consequences of them undertaking these activities are many and various. Who would like to see, for example, twelve year olds bombing up the High Street in powerful cars (which some parents would undoubtedly allow if it were legal)?
And so it is with sex. The unwelcome consequences of sex at a young age are well documented. (If you believe there are no such problems then you need read no further). The children need protection from these consequences as not every parent will provide it. The law therefore steps in to make some effort to ensure that the protection is provided in the same way as it makes it illegal for twelve year olds to drive. In neither of these examples does the law guarantee protection for the children and others who may be affected by their actions, but it does help provided children are taught what is and is not permissible. The general shortcoming of such an arrangement is that the law is not rigorously applied, and is often ignored, making youngsters believe that the law actually countenances their actions.
In summary, the problem which the law seeks to address is (like many others that impose an age limit on certain activities) is that children are often not capable of thinking through the consequences of their actions. Their parents cannot always be relied upon to do so for them, so the law tries to help to do so.