Just my own experience - couple of weeks ago I was sorting through huge pile of tubes I had accumulated over the years. I started to sort them by size, valve type and length and good to go/need patch. I inflated them and left overnight for leak testing. What I found is the tubes which are over 20-30 years old, were installed/used on the bikes they came with, etc - were still holding air just fine.
For the shape and feel - most of the far east made tubes from older days and modern cheap tubes were just OK (uneven inflation for example - like bubbling around valve, etc). But I did find couple of gems. One of them was really old Michelin tube, made for I think 19c or 20c tires and oh boy - it felt real nice to the hand, like high quality item, inflated very evenly and was just a pleasure to handle.

So I guess I am not helping much with your question but I found little to no deterioration aside from cracked patch here or there. And mind you - none of the tubes were in package, all were used at some point