A funny thing happened to Brooke Shields in her 50s. “It was like I had a superpower,” says the model-actress, who turns 60 in May. “For so long, women have been conditioned to fear aging. The messages were endless: ‘Anti-age this, turn back time on that.’ But that wasn’t my reality. It was like I had all the time in the world, and I started to ask myself what I wanted.” Here, the author (her new book, Brooke Shields Is Not Allowed to Get Old: Thoughts on Aging as a Woman, is due out in early January) and mom of two (she and her husband of 23 years, director Chris Henchy, share daughters Rowan, 21, and Grier, 18) talks to In Touch’s Lindsay Hoffman about taking control, her partnership with…