Should I replace a fluorescent bulb when it gets old, or wait until it burns out?

Update: 2018-01-17 09:01:56

There's nothing wrong with waiting until it burns out. However, fluorescent bulbs produce less light (lumens) over time. An old bulb may not only produce 25% less light than a new one, but the color changes slightly overtime. Therefore when replacing just one bulb in a multi-lamp fixture, the new bulb will be brighter and a slightly different color than the others.