Malt extract does darken with age and can develop a cooked/stale flavor in some cases. The water activity comment is correct, but mold can grow. Manufacturers take precautions in packaging but problems do occur.
For me, extract that is over a year old is beyond it's useful life. When I was extract brewing, I would not buy canned extract but only bulk extract where I knew the age of the drum it was taken from. I did not want to risk my time and effort brewing with potentially poor ingredients. It was not worth it in my opinion.
Just my 2 cents.