Are kidney bean leaves safe to eat? Are there limits as to what stage they become unsafe? Like, I can eat them young but not old? Can I eat them raw or must I cook them?
As far as I can tell, the majority of bean leaves are edible raw or cooked. They don't become unsafe, but likely become more fibrous as they mature. In this case, cooking would make eating them more pleasurable. Here are some possible uses.
Here is an edit based on the comment of @FuzzyChef, below. I had not considered the potential impact of phytohemagglutinin specifically in kidney beans. As he points out, and I agree, there does not seem to be anything definitive about the toxin in leaves. Here is a paper that discusses potential toxicity in the beans themselves. Given this information, if it were me, I would cook the leaves of the kidney bean plant.