Followup. Here's something for the engineers and chemists in the room: http://pubs.acs.org/doi/abs/10.1021/ma402297a
It's about PE, assume roughly same issues for other plastics, which will differ in their responses. But my chemist take away from this and some other articles is that 1) The only cliff is when a macrofracture becomes noticeable to the user. Before that, there's been a gradual non-linear weakening on the molecular level. And on a macro level, there's been changes in the area around the strain, paradoxically including a hardening of the material on either side and the formation of a "neck" or thinner region that will fail.
2) Plastics behave differently under low strains than high, because of something called "creep," which apparently means molecules realign along axes of strain; this reduces total strain. Can't pull this off under high strain. But they do like to form temporary molecular bridges that can be lost and (perhaps) rebuilt without permanent deformation. And at high strains, plastic can behave a lot like a fluid. Wow. Not sure what ramming into a mogul constitutes.
3) Heat and UV are bad, accelerating molecular damage. We knew this. So are lacunae and contamination on a molecular level. We surmised this.
4) Plastics appear to resist compressive forces (think sitting on a flexible barrel and having it sink slightly, bulge in the middle) better than shear or elongation forces. 5) Bad things happen when elastic deformation (meaning the plastic will return to its original shape) gives way to plastic deformation (meaning the return will be incomplete), but some plastics are capable of amazing plastic deformation before failure (think chewing gum).
I await a plastic chemist to tactfully rip me a new one over this summary. Meanwhile I've got a hunch that my clumsy answer to my vague question is that yes, boots age and lose their ability to return to their mold shape. Just like skis. No cliff, not fine until they crack or visibly fail. Still unclear though on the relative speed of aging. Is 3 years an old boot? Will Sib's new boots last that decade?