Yeah I've had salt and pepper specks at the temples since about 25, my brother too.
I think now at 34 I have a decent amount of grey at the sides. I say I think as yeah I do dye my hair once a month. I would avoid all the made for men stuff it is pretty poor, instead go with a decent hair dye for women - they're fussy as hell about their hair, so it means better quality!
Clairol perfect 10 is pretty great and looks v v realhttp://www.clairol.com/niceneasy/perfect_10/index.jsp
I figure I'll wait until about 40 and if I've enough grey then for it to look great, stop dying it. I actually look forward to a decent amount of grey so I can better wear salt and pepper charcoal flannel and other greys.
Girl wise I find girls prefer me to have grey un dyed hair, but then my GFs all prefer much older men.