By DOMINIQUE BROWNING Published: October 21, 2010NEW YORK TIMES Op-Ed It’s become a cultural norm: women of a certain age cut off their hair. MY mother hates it. My sister worries about it. My agent thinks I’m hiding behind it. A concerned friend suggests that it undermines my professional credibility. But in the middle of my life, I’m happy with it. Which is saying a lot about anything happ…