You guys need to stop debating that BS, the only proof we have about when he became hokage was that flashback, and no naruto flashback up till now was just some symbolic shit that never happened. They all happened.

Sandaime was the youngest hokage ever and he became it around ~12, remember he was one of the most powerful ninja in history and the most powerful hokage to date. Kakashi was how old again when he became a jounin? Exactly, he and Itachi were killing jounin before naruto even became a genin.
And they weren't even close to the level Sarutobi was in his prime, not even remotely close, so stop doubting that this "kid" couldn't have become it.

Kage is simply the most powerful ninja in the village, even after he reassumed the position he still had the military advisors etc.., so as a kid he would've surely had at least one person to guide him.