Why would child pornography laws have anything to do with someone pretending to be under-18? That's distasteful, sure, but objectively, people over-18 are not

Why would child pornography laws have anything to do with someone pretending to be under-18? That's distasteful, sure, but objectively, people over-18 are not children. Basic recordkeeping laws should make it easy to ensure that everyone involved is of age, even if they're sucking on a pacifier, wearing a diaper, and saying "goo goo ga ga".