I missed these 2 comments earlier, and should respond to them:
nah, they already had their summons diring that war, and Uncle Jiraiya had been training in sage mode since he was a kid.. (when he reverse summoned himself to myoboku.)
1) It's only the anime that supports these points. The anime isn't cannon.
2) Even if we pretend that it is cannon, it's still not very helpful. The summons do not appear fully grown in the flashback on the anime, and they did not use them against Hanzo (perhaps they didn't have enough Chakra left to do so). Lastly, while Jiraiya is shown to be training with sage energy since he was a kid in the anime, he could not achieve sage mode without the 2 elder frogs, who he did not have in this battle (for whatever reason)... remember, the sage mode in which Jiraiya becomes insanely more powerful? We don't even know if teen Jiraya could enter this state yet at all (even with help). So no, that doesn't help you much.
Did it state the Sannin were teens then?! I just honestly thoufh they were more like Kakashi's age which I feel would be closer to their actual prime.
Well, based on the chronology and timeline, if they weren't teens, they were still a long way off their prime. Remember, Tsunade's invention of regen techniques took Oro by surprise years later, so she clearly didn't have them yet, while Oro and Jiraiya hadn't developed alot of techniques yet either, which have been noted in this thread already.
More specifically on the chronology, while I won't break it down here, this took place at least 3 years before Jiraiya returned to the village, which was back before Jiraiya had taken a teenage Minato as a student, trained him up, etc. Minato then grew up, became Hokage for at least several years, died, and then another 16-17 years passed. Remember also that the war the Sannin were in is described as having gone on for "years" even after Jiraiya's return. So without getting into all that timeline stuff, we're already talking about a period which at a bare minimum is 25-30+ years ago. That doesn't really matter though, so much as the fact that they clearly hadn't developed alot of the techniques and abilities that made them so powerful. And we don't know the circumstances of the battle (how tired were they, how much help did Hanzo get, etc).
Hanzo was a beast who effortlessly beat Mifune back when Mifune was actually young.
Nothing about the flashback suggests it was effortless. Mifune is a weak character compared to kage level characters anyway, I wouldn't be boasting about that, and certainly not when he got beaten by Mifune the next go around.
It was his dream of finding world peace that caused him to to into hiding....and as such he lost his inner Passion of his dream and his skills dulled. Because of this he was nowhere near his power in his prime. Also note he commited Sekkupu so that he couod be sealed.....Mifune didn't entirely win (though I'm sure he probably would have).
There is no indication he was "nowhere near his prime". All we get told is before he could beat Mifune (nothing to boast of), and now he can't (embarassing for any Kage level ninja). None of this establishes Hanzo as a kage level ninja.
Hanzo was basically put up as an insanely strong shinobi....just because we never really saw him fight (and in his one fight it was a Hanzo that hadn't trained in probably several years, but was still a challenge despite that. He didn't even use any of his Jutsu really....
His failure to use jutsu is a bad thing... because he wanted to use it, but apparently can't without seals. That's pretty pathetic for a kage level guy.
The rest has been covered.
