Maybe its down to the breakdown of the anti toxidants in the fluid that prevent algae etc. Usually bottles are sealed so you are good to go.
That being said, its always good to flush and replenish your coolant. No different to an oil change in the car. It will eventually pick up some contaminants. If you were to dismantle your waterblocks you would be surprised what has built up in there.
I don't see a problem with keeping it longer than 2 years but if the seal isnt broken, you are good to go even if the coolant has sat on the shelf of the warehouse for a year. Coolant in any case is a must have commodity in the watercooling world, so I doubt very much its been on Frozen CPU shelf that long anyway.
