Glycerin is very stable. It would have to be really old before I would give it a second thought.
I ran into the following, which I thought was interesting:
GLYCERINE is the most commonly used commercial name in the United States for products whose principal component is glycerol, but it is frequently spelled GLYCERIN. More precisely, however, glycerin applies to purified commercial products containing 95% or more of glycerol. GLYCEROL (CAS registry No. 56-81-5; NIOSH No. MA8050000) refers to the chemical compound 1,2.3-propanetriol, CH2OHCHOHCH2OH, and to the anhydrous content in a glycerine product or in a formulation. Outside the United States. especially in Europe, glycerol is a much more broadly applied term, being employed much in the same manner as glycerine is in the United States. Glycerol in European usage may pertain to any grade of glycerine, including crude.