Karada wa Kodomo, Nakami wa Ossan no Nariagari Boukenki

Grey Millard was born as the third son of the poorest aristocrat in the empire. In the Millard territory of the empire lives its feudal lord, Grey's father. However, his father is being controlled by his fiendish stepmother. Disliking this kind of trouble, Grey immediately determined to leave the Millard territory. Until Grey severs his relationship with his parents at 13, he devotes his entirety on self-improvement, ignoring those suffering under his stepmother's tyrannical rule. But...

This is the adventure of a boy with the mind of a middle-aged man.

(Source: Shousetsuka ni Narou, translated)