system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does paper yellow with age?

I was originally going to post something to the cooking subreddit about old recipes on old, yellowed notecards. 
But.. I don't know how old they are and assumed they where old because of their yellowed, stiff feeling. Then I realized I didn't know why paper yellows at all. 
Is it like wood aging?
gpt
There are two main components to wood. Cellulose structures which make up the cells, which is what most paper makers are after, and lignin which is what makes trees rigid. Lignin basically glues the tree's cells together. Some paper is made by just grinding up trees and bleaching everything. Other types of processes use chemicals to dissolve the lignin to keep only cellulose as the final product. 

Anyways, lignin is prone to oxidation, and especially with paper grades like newspapers, yellow over time.