FIFTEEN-YEAR-OLD GIRLS don't want advice from their mothers. It's a truism as old as time, and yet for every parent, a bitter pill. When it comes to crafting and caring for their emergent selves, teenagers look to their phones, to brands, to celebrities. In this moment, they are finding all three in Selena Gomez, who, with 427 million followers, is the most followed woman on Instagram. This is despite the fact that she famously logged off social media for four years, returning only in the past year, because it was wreaking havoc on her mental health. When I ask my own teenager and her friends—musical-theater nerds, athletes, Black, brown, and white girls—to explain the singer, actor, and cosmetics founder's allure, one says succinctly, “Selena Gomez doesn't make me feel less-than.”…