this linked from this says after 30 years pinto beans will cook and will be edible (80% acceptance for emergency use).

If they won't absorb water, you're supposed to grind them up

Why they get hard? Heat, oxygen and humidity
Originally Posted By: http://onlinelibrary.wiley.com/doi/10.1002/jsfa.2822/abstract
improper post-harvest storage conditions, such as high temperature (30-40°C) and high humidity (>75%)
Soaking for 48 h also leads to hardness