system
Given the following post, respond as if you're commenting on Reddit:
human
Why, if we get a new set of skin every 80 days or so, does skin ever become wrinkled and old? 

Cells replace each other, so why does skin age if they are new cells?
gpt
Collagen and elastase degradation in the skin matrix.  These proteins give skin its flexible nature.  Over time the protein is no longer secreted as readily by fibroblasts and the existing matrix begin to degrade, resulting in looser skin.