Sapor II, King of Kings of Iran and of Non Iran, went from literally not existing to monarch in much less than the 9 years that Grant rose from his lowest status to be president, being crowned in the same year he was born, and thus less than two years after conception - according to legend he was crowned in his mother's womb.
Of course Sapor didn't do anything to become monarch.
According to Gildas in The Ruin of Britain Maglocunnus the Island Dragon usurped his first throne about AD 500 when in earliest adolescence, and thus in a much shorter time after birth than in the case of Grant.
Elagablus, a hereditary high priest, son of a senator, descended from kings and related to emperors, usurped the throne of the Roman Empire in 218 in a bloody Civil War aged 14, though older family members did most of the planning for him.
In 238 AD, the Year of the Six Emperors, the civil wars and assassinations resulted in Gordian III, related to Gordian I and Gordian III, becoming sole emperor at the age of 13, though it is unknown what if anything he personally did to gain the throne.
Himiko or Pimiko, Queen of Wa, who lived in Yamatai somewhere in Japan, died in 248. According to Chinese records:
When Himiko passed away, a great mound was raised, more than a hundred paces in diameter. Over a hundred male and female attendants followed her to the grave. Then a king was placed on the throne, but the people would not obey him. Assassination and murder followed; more than one thousand were thus slain. A relative of Himiko named Iyo [壹與], a girl of thirteen, was [then] made queen and order was restored. Chêng issued a proclamation to the effect that Iyo was the ruler. (tr. Tsunoda 1951:16)
So Iyo or Toyo became queen age 13, though there is no record of how much she did to gain the throne.