From the time they are first fired they gradually start changing the color spectrum they produce, eventually getting further and further away from the desired color range and closer into the red area. Corals in general process the blue end (400-470nm i think is about right) of the spectrum to synthesize into food, on the plus side if you want to grow hair algae old lighting is a great way to encourage it. The actual amount of light output being produced will also degrade over time as well.

Wait a year and put a new tube alongside an old one... you'll see the difference, swap them out for your tank and use the old tubes in the garage or some place usefull.