system
Given the following post, respond as if you're commenting on Reddit:
human
If our bodies replace all their cells between 7-10 years why do I still have scars from when I was little?

I know that the 7-10 years may not be accurate for every cell.  But even just for skin, why do I still have scars from 20 years ago?  That's not an actual DNA change, right?
gpt
Scar tissue is mostly **not** cellular.  It's a dense, fibrous network of collagen.  

Collagen is a structural protein that features most prominently in connective tissues.  The collagen network in a scar is different than that in other places where it is found; it is much more dense and the fibers align with each other, acting as a barrier that cuts off the wounded tissue from the surrounding healthy tissue.