[JR1427]

If hair greying was mostly caused by UV damage, I would expect that the pattern of greying would be even, and begin on the top of the head.

In contrast (based on my own unscientific observations!) greying typically begins in small areas, and often on the temples - not what I'd expect if caused by UV damage.

[gib444]

Now I'm curious why it starts on the temples (it's where mine started too, then my fringe)