shelf life varies for many reasons ranging from temperature, proper sealing of the container, and a gray area vendors generally lack focus on is the actual manufacture of the material itself. Vendors act as a reseller for the material, they buy in bulk then repackage the raw material into smaller amounts for retail sales. The "date" on the label is likely the repackage date, not the manufacture date. Some vendors will have material sit in a warehouse for weeks to months and ive even seen longer, before being repackaged for individual retail sale.
As mentioned above, only buy what you can use or if you know it will be used in the direct future.
From personal experience, I just used silicone with a manufactures date of 2009 with no issues. I did however use new catalyst. The consistency was that of peanut butter but thats what was needed for the project. The initial consistency was that of a thick shampoo. Another trick is to use some Vicure. Thats actually to "kick" the silicone to cure as fast as needed. Ive had instances where there was literally minutes to make a mold and have parts from it. Using Vicure will reduce the molds life so its not recommended for anything other than "RIGHT NOW" parts. But, as mentioned, it works well with old silicone.