Pharrell Williams has revealed the secret to his eternally youthful skin. The former N.E.R.D. singer turned 51 in April and has spent years renowned for his young look – and has now told how one simple beauty treatment is key for him to fighting wrinkles. He told People when asked for his beauty secret: “Believe it or not, exfoliation, man. Exfoliation is the key.” But 13-time Grammy winner Pharrell added it wasn’t quite as simple as scrubbing his face. He added: “Now, there is a routine, and you can find them online, but I think probably the most active portion of it all… no, it’s two. “It’…