Neither is a teen freely sending a nude selfie of themselves to their teen SO.
Many family photo albums feature naked kids in various stupid situations that were just thought to be cute and funny at the time, which would nowadays melt peoples brains if they'd hear about it, and certainly would fall under the same laws that would punish the teen in the other example.
UK proposes selfie-based, AI age verification system for porn sites (www.techspot.com)