When I was about 31. Okay, but seriously, I am one of the only women I know in my age group with as much grey hair as I have, and I did let it go for about 2 years. I liked it. I have nice-ish hair and people tend to compliment me on my hair anyhow, but I got a surprising lot of compliments on the color when it was natural. I enjoyed having zero maintenance hair color after dying my hair since the age of 13 and I was surprised to see how dark and full of red it is, I never really knew that. I color my hair again now, but I look forward to letting it go grey once it’s more than 50%.