When I read a line such as "I would never want to see an underage person naked.. just the thought feels dirty..." I realize that our society has promoted a false taboo and in doing so, unwarranted shame. As shame is a precursor to problematic self imagery and self degradation...the normal becomes sinful , the sinful becomes temptation and temptation to furthered shame. (And sometimes actions.)
Those that are learning in their first society (family) that the human body is not something to feel ashamed about, are able to see the body as the beautiful, even miraculous temple of the soul ...of which it is. These folk are not going to feel degraded, hence degrade others at the appearance nor thought of an unclothed body. They are not going to have an unbalanced sense of proportion and rebel by being immodest nor promoting or forcing others to be either. Not so true with those that feel 'dirty' about the natural form of a human being ...they tend to 'think dirty', talk dirty and do dirty 'things'. In other words, their repressive beliefs end up as a force within their morality that requires control. Out of control and ...?
Finally, I admit that I have seen a fair share of naked children. Having seven children of my own, each of them beautiful from the moment of their naked entrance into this world (through the diapered through bathed years as well) and beyond ...I have never seen them as anything but clean, healthy, beautiful gifts of God. To even think of them as dirty would be a testimony of a mind/soul that has been damaged.
To think of a person being 'under-aged' or 'of-age' seems a category of licentious thinking and self permissiveness of a person that thinks pornographically.
In terms of art, I do not believe that pictures of humans without clothing at any age, specific nor generally, is a pornographic undertaking.
Any actual recording of abuse that is presented as art, is, in my opinion, simply abuse and not art at all..as it is not artifice, but documentation of illegal activity. A photograph of a 12 or 13 year old (to stay on topic) swimming naked is not what I mean. A girl of the same age being involved in intercourse, however, is exactly the area of abuse that I am referring to.