Sage Mode is one of the most powerful tools in Naruto's arsenal, but when exactly did he learn how to harness it? The short answer would be in 'Sage Naruto!!' chapter 418, volume 45 of the manga. The long answer? Naruto has several Sage Modes, and not just the ones whose only difference stem from if you get your powers from a snake cave or a frog mountain.
Naruto is a substantial series with 72 manga volumes, 11 feature films, and a spin-off series featuring his son, Boruto. With his series reaching its epic conclusion, Naruto has mastered, from a narrative point, all techniques that he set out to achieve. There should be nothing to reveal left, but much like a world-renowned shinobi, Masashi Kishimoto has a few tricks up his sleeve. Like will this Sage transformation play into future battles? And what reptile makes for a better teacher?
Naruto, in his first series, was destined for greatness from the get-go as his father implanted the nine-tailed fox demon that was rampaging their village into the infant Naruto. Be he blessed or cursed with these powers, Naruto struggles through adversity as he strives to become Hokage, or strongest ninja of the Hidden Leaf Village. Naruto, along with his friends Sauske and Sakura, fight and train their own ways to achieve their goals. Inherently gifted, Naruto quickly perfected most techniques of jutstu, chakra absorption and transference, Rasengan, genjutsu, taijustu, and, of course, the coveted Sexy Technique (mostly used for the anime’s filler plot). However, Naruto would only learn Senjutsu, the justu needed for going Sage Mode, after experiencing a tragic loss.
Being the natural prodigy he is, Naruto surpassed many of his former masters on his path to become Hokage. First was Iruka, then Anko, then Kakashi, he trained under Guy-sensei and the ‘ghost’ of the Fourth Hokage, Namizake Minato, before studying how to master Senjutsu under Jiraiya. Jiraiya was the closest to Naruto personality-wise (besides Guy-Sensei), and was the one to have trained Naruto’s narrative foil, Nagato, only to be killed by the Six Paths Of Pain he taught to his first protégé. After his master’s tragic passing, Naruto sought revenge, and frogs.
Whichever philosophy you subscribe to - frogs or snakes - the differences in these methods matter in Naruto: Shippuden. After Jiraiya’s death, Naruto sought training directly from the frog itself, and only then was he able to balance the forces within him and achieve Perfect Toad Sage Mode. Jiraiya could only ever achieve Imperfect Sage Mode due to lack of balance and insufficient chakra, though Naruto did have around 9 other helping hands. He even worked around having two celestial animals (fox and frog) inhabit his body at once by making shadow clones to gather power and pulls a reverse summons to receive both cosmic creature’s full chakra flow at once.
Naruto can fuse these types of chakras together if he is hurt with something like a chakra-stopping blade, creating a god-like form that comes with Xs in Naruto’s eyes. Not to say achieving Sage Mode is easy - it’s either get hit by a frog with a staff or get selected by being bitten by a poisonous snake. This is something that Sauske could only hope to achieve, but as he had already received a portion of the Ryūchi Cave’s power from Orochimaru, he felt he could hold his own against any enemy. Through hardships he finds success, and in his training and Naruto proves that ninjas are "People Who Endure!"