Star Wars has pieced together when Palpatine became Darth Plagueis’ Sith apprentice. While Palpatine never actually said, in the movies, that he was the apprentice mentioned in the “tragedy of Darth Plagueis the Wise,” it is well established that Sidious was trained by Plagueis. Likewise, Star Wars has made clear that Darth Sidious killed Darth Plagueis, fulfilling the Sith Rule of Two and becoming “all the Sith” for decades to come.
In terms of the Star Wars movies, chronologically, Sidious’ journey begins in Star Wars: Episode I - The Phantom Menace. However, Palpatine’s Star Wars backstory begins decades before the events of the Skywalker saga. For example, Palpatine met Plagueis well before The Phantom Menace. Thanks to Star Wars Timelines by Kristin Baver, Jason Fry, Cole Horton, Amy Richau, and Clayton Sandell, Palpatine's past is now less of a mystery.
Star Wars Has Hinted Plagueis Came To Naboo When Palpatine Was A Teenager The first major hint that Darth Plagueis came to Naboo when Palpatine was a teenager can be found in Star Wars Timelines’ Naboo portion. The Star Wars reference material mentions Damask Holdings building a plasma refinery complex on Naboo in 65 BBY. Though this detail originates from the FFF roleplaying guide Nexus of Power, it could tie into James Luceno’s Darth Plagueis. The Legends Darth Plagueis novel established that Hego Damask was Plagueis’ alter ego. Therefore, Darth Plagueis might have been operating on Naboo since as early as 65 BBY. Should that be the case, then Palpatine was around 19 years old when he met Plagueis.
James Luceno’s Tarkin novel also supports the idea that Darth Plagueis recruited Palpatine when the latter was 19 years old. Tarkin was 20 years old when he met Palpatine. During one of their first exchangers, Palpatine mentioned a “teacher.” Palpatine compared his own career to Tarkin’s, noting that he was younger than Tarkin when a certain person helped him realize what he “wanted in life.” Palpatine also mentions that this teacher helped him “attain” whatever he wanted. If Plagueis arrived on Naboo around 65 BBY, then Palpatine was indeed younger than 20-year-old Tarkin when the future Sith Lord met his master.
How Long Did Palpatine Train Before He Killed His Master? According to Star Wars Timelines, Palpatine joined the Senate in 52 BBY. Assuming that Darth Plagueis arrived on Naboo and recruited Palpatine around 65 BBY, Sidious trained as a Sith for 13 years before becoming a politician. Darth Plagueis was still alive when Palpatine joined the Senate. In Star Wars: The Secrets of the Sith by Marc Sumerak, in which Palpatine narrates key moments of his life, Sidious mentions what his goals were after joining the Senate. Palpatine wanted to explore the galaxy’s fragility as a politician, but he also wanted to become “the true master of the Sith.”