What causes grey hairs on different parts of your body? (ex. underarms,eyelashes,etc)?

Your individual hair shafts simply run out of melanin. Melanin is what pigments (colors) your skin and hair. The darker the skin and hair, the more melanin you have. Hair shafts run out of melanin at different rates, so you get different areas of gray at different times depending on each individual hair shaft. You can find out more about it at the Mayo Clinic site.

What causes grey hairs on different parts of your body? (ex. underarms,eyelashes,etc)?

I dont know ive always wondered the same darn thing thats a good question i think you body just stops making as much pigmentation thats why even for blacks when they get older the skin always brightens up by like 2 shades. And someone may have brown eyes there whole life but they'll turn more so hazelish.