After sunscreen, the skincare ingredient dermatologists most often recommend to keep signs of aging at bay is retinol. But what is this must-have fountain of youth? Bazaar asked three powerhouse New York City dermatologists, Dr. Shereene Idriss, Dr. Melissa Kanchanapoomi Levin, and Dr. Joshua Zeichner to explain all there is to know about retinol and why you need to make it part of your regular skincare routine pronto.