The porn video tag "young-big" seems to refer to two distinct characteristics: 1) young individuals, and 2) a focus on large body parts or features. In this context, the term "young" is likely referring to performers who are relatively young in age (often under 30), and the term "big" refers to large body parts such as breasts, penises, or buttocks. This tag can be used to describe content featuring young individuals with notably large physical features.