Are NOS phono cartridges best avoided?

I have read that the compliant suspension components in phono cartridges are subject to deterioration/hardening as time passes. Is this strictly a function of time or does exposure to atmospheric conditions and polutants factor in significantly? I occasionally see "new old stock" cartridges for sale/auction that were good performers at the time of manufacture 10-20 years ago. Are these best avoided or are they still useful if they have been sealed in factory packaging? Anyone have experience with this sort of thing? Thanks, Will.
I have gotten several of the old GAS "Sleeping Beauty" MC cartridges for $50 a pop. I love'em, they sound great, and I have had no problems whatsoever. When new, they were around $150.
Most cartridges do suffer a "shelf-life" effect. This is due to the suspension loosing compliance because the elastic material ages when exposed to the atmosphere. I suspect that storage in the correct inert gas environment could extend this, however, I doubt that anyone has gone to these efforts. I would consider a cartridge that has zero running hours and a moderate age (perhaps up to 5 years) a purchase option, but would avoid ones that are considerably older.