Component Shelf Life

I'm conducting research over the shelf life of components, the effects of ageing and the reason why different suppliers define different shelf life rules.

We, as a factory, place a lot of Intel (former Altera) components and those are delivered with a shelf life specified.
Other research concludes shelf life has, in lead-free finishes, no (big) influence on solderability and thus plays no significant role.

What I'm looking for is the rationale behind a shelf life definition: how is it defined and what are effects on the components if we exceed this shelf life?

The reason for my research is to define a clear, well-founded conclusion how we, as an EMS factory or even as an Industry, define and handle shelf life.

I'm looking forward to your response, hopefully some (research) papers are available.