Well, this is a bit of a grey area. One problem is that there is no official definition of pornography and other such things. I believe the explanation given in some major court cases was, in essence: "I don't know how to describe it, but I'll know it when I see it."
Which can be a problem in these types of discussions. There no clear definition.
However, I think the general rule of child porn is thus:
~The people involved in the acts as recorded are guilty for actually creating it.
~Those who view it are just as guilty as they drive the production and distribution. Without them, it would not exist.
Now, if we're discussing sexualized young people in film and art, that's not quite as easy. For one, no one is being harmed. However, as far as I know, there is nothing in anime, manga, or games (not counting hentai) that would qualify as child porn. For one, they don't have sex with anyone and aren't shown naked in sexual ways. The only nudity I can even think of tends to be incidental due to bathing or such.
I'm not going to say that using some questionable things from legitimate sources as evidence when a person has child porn is bad. It shows a interest in that sort of thing. And helps to paint a picture.
That being said, reading a manga that sexualizes a 15 year old, or even a loli, a little isn't grounds for Child Porn accusations. It would just paint a bigger picture. Maybe the person was interested in it due to the interest in the porn. But it wouldn't work to say the porn was a result of that interest in the anime/manga. If that makes sense.