Plastic degrades over time. Sun exposure and mechanical stresses accelerate this process. This is the reason you have date stamps on plastic parts (see image below.
Plastic does not necessarily get weaker. It gets brittle. This means that it can fail in new and exciting ways. My personal preference is no more than 5 years for "mission critical" parts. Do people still use older boots and bindings? Sure. Is it a good idea? Last I checked, emergency room visits are far more expensive than new bindings.
My $0.02