The Permian/Triassic mass extinction seems to have been the turning point. The creatures that have not changed much since that earlier time are the creatures that don't age. But when life revived at the end of the Triassic, the world was more competitive, eco-systems were more integrated, and specialization became more common. Aging allows specialization, we can see that most obviously with birds, which need to maintain a specific weight and body mass and ratio of wing span to body mass. A bird that simply grew and grew and grew, like a tortoise, would have a changing ratio of wingspan to body mass and therefore it would soon lose its ability to fly. The creatures that age are those that have a fixed adult body shape. By contrast, the tortoises and reptiles and fish that live forever also seem to grow and grow and grow -- their body mass to external ratio changes over time, therefore they are less specialized.