Interesting thread...
I'm almost 31, and have "white streaks" coming through - at the moment they are sparse enough that they don't look out of place, but I do colour my hair about 3-4 times a year, and will keep this up as darker hair suits me more than lighter hair does - lighter hair makes me look unwell. So its not the grey that makes me want to continue colouring - under my current colour is a lighter blonde that I experimented with so I'm covering it until its all gone!
But, my brothers (24 & 26) are quite grey - they started going grey in their late teens. 1 colours his hair (his girlfriend is the beauty therapist so they, as a couple, are both more aesthetically aware than a lot of people around here), the other one just keeps it short and because he is so pale naturally you don't notice it. So I'm very pleased that I've had quite delayed greying in comparison to the boys. Interestingly, they both have VERY noticeable receding hair lines so I can see them being grey and bald before they are 40...
My Nan has pure white hair that is just glorious on her! She has lovely blue eyes and keeps her hair short. So, if my hair ends up like hers (as opposed to my dads that is more a true grey), I'll happily sport it au naturale when its all white...