Well in the first manga page when he was with the future sannins, they were literally looking at Hiruzen's stone face with the other Hokages, so that should be enough to give it away that he was a sensei and hokage just like Hashirama and Tobirama were.
The second url I posted showed a younger Hiruzen being appointed Hokage by Tobirama during the Kinkaku forces going after him. So Idk when it said Hiruzen became Hokage but 31 years from what exactly?
How old was he during Danzo's flashback?
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