One of the biggest questions about the period between the Star Wars prequels and the beginning of Star Wars: Episode IV – A New Hope is the Empire’s unspoken transition in its military which saw the Clone Troopers replaced with the (non-clone) Stormtroopers from Episodes IV-VI and Rogue One: A Star Wars Story. There has yet to be a formal answer in any of the movies or television shows, but non-canon sources have come up with some elegant explanations. However, a canon explanation does exist, covering not only when the Empire replaced the clones with Stormtroopers, but why.
Clues can be seen as early as Star Wars: Episode II – Attack of the Clones, as the Kamino leader Lama Su explains the clones’ life cycle to Obi-Wan Kenobi: “[growth acceleration] is essential. Otherwise, a mature clone would take a lifetime to grow. Now we can do it in half the time.” The statement implies that clones age far more quickly than non-clones, and thus need to be either replaced or discarded within a few years of their use. That detail fits with non-canonical explanations, such as the Star Wars: Battlefront II video game which posits that a revolt among the clones prompted the Emperor to change his recruiting methods.