First grade starts at 6 in North America, 5 if your born late in the year, kindergarten and pre k might get you as early as 4, but this kid was 10 that 5th and 4th grade, my parents wanted me out and about as much as possible at that age, just being home at dark and for dinner.
You don’t need 51% of the world’s power though, just 51% of the power of people who care about how the system works. Most people using block chain cryptos don’t care at all, so the threshold is a tiny percentage of the user base.