The Mandalorian offers the best explanation for why the Jedi Padawans begin their training during their younger years. During the Star Wars prequels, the Jedi Order was in the process of training Jedi younglings - a group of Force-sensitive children from across the galaxy - to become the next generation of peacekeepers in the galaxy. That included Grogu, who miraculously survived Order 66 and Anakin Skywalker's rampage after his full evil turn in Star Wars: Episode III — Revenge of the Sith.
It was never revealed why the Jedi develop their abilities during their youth. This was the norm in the prequel trilogy, which was set during the Republic era before the Sith led by Emperor Palpatine successfully slaughtered most Jedi and drove the remaining ones into hiding. Even Anakin Skywalker, who was taken in by Qui-Gon Jinn from Tatooine after believing that he was the Chosen One, had to bid his mother goodbye in order to begin his training immediately - despite the Jedi Council believing Anakin was too old to be trained.
Lucasfilm took years to give a proper explanation regarding this matter via The Mandalorian season 2. As his primary mission, Din Djarin was tasked to reunite Grogu with the Jedi, which led them to Ahsoka Tano, Anakin Skywalker's former Padawan. Mando thought that this was the end of his responsibility to Grogu, as Ahsoka would take him with open arms, especially after learning that he's a survivor of Order 66. But Ahsoka refused, citing Grogu's palpable fears and attachment to Din make him susceptible to the dark side. Instead of honing Grogu's skills and risking his corruption, Ahsoka suggested letting him go untrained and wait until his powers fade over time. This effectively reveals why the Jedi started training Padawans early on, as leaving their powers untapped for an extended period of time can ultimately lead them to lose their Force sensitivity.
Two prominent Star Wars heroes, however, do not fit into this idea. Both Luke Skywalker and Rey started their training fairly late compared to the younglings in the prequels. Obi-Wan Kenobi did give him some pointers during his initial bout with the Empire in the original Star Wars movie, but Luke didn't begin proper training until The Empire Strikes Back under Yoda, when he was 22 years old. Rey, on the other hand, started her formal training with Luke in Star Wars: The Last Jedi when she was 19 years old. Despite this, both became powerful in the Force and are among the most capable Jedi in the history of Star Wars. So, the pair may be the exception to the rule because of their innate strong connection to the Force.
While it's been a couple of decades since Grogu's Jedi training was interrupted by the rise of the Sith, he appears to still be strong with the Force. Given this, it's curious if he falls in the same category as Luke and Rey who were able to retain their Force-capabilities despite not actively using them for an extended period of time. Viewers will have to wait and see what lies ahead for Grogu, as he officially restarts training with Luke, which might be something that's tackled in The Mandalorian season 3.
