Not really a child
I am contrasting this defense from the defense of virtual child pornography. Thus, file does depict a real person but that person does not meet the definition of a child in that jurisdiction. Luckily, I think most prosecutors will not take a borderline case to trial. However, it has happened. (Lupe Fuentes). Besides prosecutorial discretion, one of the reasons that this is rare is that there is a database of known child pornography that is maintained by law enforcement across the country. These files have been authenticated as child pornography in other courts and in some cases, law enforcement can actually prove who the child was and how old he/she was when the contraband file was created. Thus, this is generally not an issue.
The more likely scenario is that some of the files will be borderline and the State may try to bring these videos in to increase the number of files presented to the jury. A pre-trial motion on this issue should solve this problem. Doctors are sometimes brought into to testify as the anatomy of the child to prove that the person depicted is of a certain age.