Chris really hit the nail on the head! The life expectancy of your suit, even if cared for meticulously, depends on the material that it was originally made of.
The old Rubatex material from the 70s and 80s that was affectionately dubbed "iron ware" by many dive shop owners, was stiff but extremely durable. With these suits, custom was almost mandatory. I still see some of these old wetsuits used in rentals that were made with the stuff way back in the early 80s.
With the advent of the "soft stretch" materials in the mid 80s came a drastic life expectancy decrease.
Neoprene is comprised of thousands of tiny bubbles of some gas. The old iron wear used inert nitrogen so it did nothing to damage the rubber itself. The only damage came from the repeated compressions and decompressions of diving. Because each tiny bubble was filled with nitrogen, they didn't break down as easily.
Soft stretch, on the other hand, was/is chemically blown. That means that the chemical process can still be going on to some extent on a suit that is already in use weakening the bubble walls. When those tiny bubbles are compressed, a certain percentage of them will stick together and remain collapsed while some others will burst. You can end up with a suit that looks great still, but has no insulation/bouyancy value.
Old iron wear will get you probably in excess of 500 dives before it is worn out while the newer materials will only get you about 150-175 dives to get to the same state.
Just my $.02
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Ray
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