My experience was years, not months.
They will usually have a shorter useful life when one starts to wear them.
But they get used, apparently. There is a difference in irregularly used and stored in situ.
There is also a difference in when a shoe is made as the materials and glue may be different, as well as in which factory a shoe is made.
Returning to the same store I tried another pair of the same make and model shoe and they did not fit well. Following up with the company they informed me there were two factories that made the shoe, and how to tell which was which. I found the make and model from ‘the factory’ and it fit perfectly. That could seem irrelevant but hopefully makes a point in how many opportunities there are for something to be ‘different’ even using the same patterns, lasts, and processes.
Your problem could be the result of age (your original question) or something so simple as a weak batch of glue, a process failure (heat or humidity during manufacture) or a substandard glue batch that was not detected.
As an aside, a separate discussion to whether shoes have a shelf life this is how manufacturers should stand behind their products!
