Elemental iodine in solution with iodide forms tri-iodide, which is perfectly fine to consume. This is what Lugol's solution is and it has an unlimited shelf life.
It is fine to consume, but the part of the iodide that has been converted into iodine can no longer be transformed into a useful form inside the human body.
Therefore any reserve of iodide will provide a smaller dose than expected when it is too old, unless it was stored in special conditions, e.g. in nitrogen without contact with air.
