You are basing this only on the fact that their parents graduated at that age, but now we know Himawari's age and Boruto is not more than 3-4 years older than her. They always looked younger to me anyways, 10 is reasonable.
To me Boruto looks 10 and Himawari 6, all manga and movie evidence supports this. There is no way they are 6 years apart, Himawari was running with Boruto when he was 4-5.
Their parents, the generation before their parents...I'm guessing from what we've seen in the series that 12 is about the age they graduate from the academy. And how do you know Boruto isn't 3-4 years older than Himawari? How is it hard for you to believe that he's 12 and she's 6? I don't get it.
We only know about their parents generation, and there are always exceptions like Itachi and Kakashi who not only become Genin but all the way to Jounin before 12. The theme of "the newer generation surpassing the old" is constantly being enforced and these new kids graduating at a younger age than their parents would be expected.
Boruto is a shortie like his pa.
What about Itachi who graduated at 7?
They were considered prodigies and advanced faster than everyone else. It was rare for people to leave the academy that young. The rest of Kakashi's age group didn't leave the academy until way after. Then we have Rock Lee, Neji and Tenten who also graduated at 12 as they were 13 when they entered the Chunin exams and one class ahead of Team 7 and the rest, who were also all 12 when they graduated..
You never know...It could be a flashback showing the last straw before Boruto started to resent the Hokage tittle.
