Personally, I do not think there should be an age to determine whether someone is a "child" or not. I know people who were mature enough to be an "adult" at 13, and others who should be considered children at age 40. As far as pornography goes, though, there was in interesting incident a few years back where a painting of Baby Jesus
was shipped from Rome to the US, and was confiscated for being child pornography. (enough said)
Kiddi porn can be good in a way, it keeps the pedeophiles busy wanking at pictures instead of going after children (although I doubt this is a fair argument.)
Being 16, I like finding pictures of kids in my age range (14-17), because that's who I'm sexually attracted to. At 9, a child isn't sexually attracted to anyone, and therefore should not be photographed nude. It's that simple. Untill children figure out their sexuality, I think it is totally immoral to use them as objects for someone's masturbation excercises.
Now because legality is all about rules, there MUST be an age limit. Personally I think it should be 16, because I think that children have developed enough to make their own decisions at that point. If a 16 year old is forced into making porn, how is that different than a 21 year old being forced? It's not. The only difference is that acording to our laws, the 16 year old doesn't have a mind of his own.