Food rots every day over a certain number of bushels I believe. Not sure what the exact number is.
Also, more food you have, more food you will lose to rot.
You can't tell which food is older and which isn't. It doesn't matter as long as you are buying it off of the market. The problem is when your food doesn't sell and return to you, rot will take away food equivalent to how many days your food was on the market I believe.
