The 50,000 hour rating is an estimate for running the LED at maximum rated brightness, properly heat-sinked, until it dims to 70% of its initial output. Manufactures don't test the full 50,000 hours. They test to something like 6,000 hours, then use formulas to estimate out to 50,000 hours or whatever.
If you look up Cree's LM-80 testing, you'll see results for all their LEDs. The interesting thing I note from that report, is that current (brightness) has very little effect on lifetime. It's almost entirely due to heat at the LED junction point. If you keep the LED under 85C, it will probably outlast the 50,000 hours. If you keep it down around 55C, it will essentially last forever. Temperature above 105C kills the LED much faster than the 50,000 hour rating.
The conclusion is that heat-sinking is everything, and a way for that heat to get out of the light. This is why home LED bulbs often die too soon, because they're trapped in a housing that just builds up heat. Proper flashlights don't suffer from that problem.