Time- both in age and usage- have a impact on the light output of a bulb and HID's are no exception. Just as with a incandescent bulb where the filament wears out and spontaneously breaks, HID bulbs will gradually dim over time as the the electrodes wear and the salts in the bulb capsule degrade. Some HID bulbs will take on a "pinkish" hue as they near "end of life." After 10 years it probably is time to consider replacing them.

Also, in what condition are your headlamp lenses? Extensive oxidation of the plastic can dramatically decrease output.

 

Interesting, thanks for the info. My lens are still in great condition, I take care of them. This pic was taken today, sun is shinning too much on the lense. You can kinda see how clear it is.

 

Only @ 120k on my 2012 G37xS, but my original 2003 G35 is @ 290k miles and still running strong, engine is good and transmission is still good. But showing some oil leaks now on the engine.