Why hasn't my LED burned out by now?
Properly designed, built and used, today's LEDs have incredibly long lives and the wearout mechanisms are not catastrophic in nature, so instead of using incandescent lamp MTTF statistics, a luminosity percentage (70%) is often used to define the lifetime- this doesn't mean that the LED burns out, it means that the light out put is only 70% of what it was when brand new.  Because the change is slow, you normally don't notice it.  

For white light applications this makes sense because after a period of time the light works but may be too dim to be useful in the application such as a flashlight or reading lamp.  

FWIW incandescent lamps are prone to something similar in that the filament would slowly evaporate leaving an dark coating on the glass shell, dimming the light.  They are able to compensate a little bit because the filament glows a bit brighter due to the increased resistance (until it breaks).