Eh. Generations are defined by a lot more than what clothes someone wore or what TV shows were being broadcast. Those things move quickly. Generations are usually marked by larger cultural touchstones.
There are quite a few ways to try and slice the Millennial/Gen Z divide, for instance. An easy-on-paper ones are things like what generation your parents belonged to (Boomers/Gen X, respectively), for instance, though that just kind of pushes the issue back to a different generational divide. Or there's the "do you remember the world before 9/11 happened?" metric. These point to differences in parenting, or differences in the larger socio-political culture within which one had their formative years, and they're far, far wider reaching than fast fashion.