Like wrinkles, grey hairs are a visible sign of ageing—and let's be clear, there is nothing wrong with that. It's a part of life, and a beautiful piece at that. But if you've ever wondered what causes hair to change colour over time, trust me, you're not alone! Intrigued? Here's what you need to know:When do most women go grey?Most women who are in their 30s should see a few grey hairs. But by the …