Eating old peanut butter will not make you sick unless it has mold in it or other bacteria in it.
However as long as the peanut butter smells good, has no mold etc then it's likely okay and safe to eat.
I've eaten old peanut butter before and never gotten sick from it.
Peanut butter can get hard and start losing it's flavor the older it gets but it's okay to eat expired peanut butter.
Peanut Butter can give you diarrhea if you eat too much of the peanut butter.
However peanut butter when eaten in moderation should not give you diarrhea unless you have issues digesting it because doctors recommend that when you are experiencing diarrhea that you eat some peanut butter.
Eating some peanut butter when you're experiencing diarrhea is supposed to help stop or slow down the diarrhea.
But some people do have digestion issues and eating the peanut butter may actually lead to diarrhea.
My own sister gets diarrhea when eating too much peanut butter and her stomach also begins to hurt as a result of eating the peanut butter.
Peanut butter contains fiber which can make you poop and make your stomach feel bloated as well.
Peanut butter is healthy though when eaten in moderation but it's also possible for the peanut butter to cause issues when you eat too much of it.